Sorry for your loss.
I'm a Paramedic, but I can't speak to specifics of your mother obviously. I can talk about some related factors with my experiences and some of it may fit in to what you experienced. I second what others have said, you can always email the trust and ask for a bit more information about that day. A patient report form will have been completed that will document very clinically what happened, whether this is ever released to family i'm not sure but if you explain you've had some persisting questions that have bothered you I would certainly expect them to try and help you with answers.
I'll start by saying out of hospital cardiac arrests are very rarely survivable. Only about 4% will ever leave hospital and this tends to be children/the 40-50 yr olds who had a heart attack but otherwise no medical history. Anyone over 65yrs, especially if they're diagnosed with a heart/lung or kidney condition - it's exceptionally rare to survive an out of hospital cardiac arrest. This sounds odd, but you have to be quite 'fit and healthy' to survive a cardiac arrest - I say it sounds odd because you don't normally associate people who are fit and healthy having cardiac arrests - but what can happen in these groups is their heart just has a brief electrical abnormality/small obstruction in blood flow that stops it working - but this is something relatively easy to fix if they survive to hospital and the doctors can make their repairs and recovery is possible.
CPR doesn't 'cure' anything, it's a temporary measure to maintain blood flow to the brain/body so it survives for Doctors to treat the cause. Even when successful at restarting the heart. So in a patient who's got chronic health conditions/frailty which has deteriorated to the point of cardiac arrest even when CPR is successful all of those problems remain, plus now the body has been severely weakened by the event itself.
Ambulance triage/resourcing - when it's recognised in a 999 call that a patient is in cardiac arrest/not breathing they'll try and resource it with 2 ambulances and a specialist whom normally works on a car but there's the helicopter too. The only time they don't is if the caller also says the patient is cold and stiff/report there is a DNACPR in place - the call taker can't ask this but if the caller volunteers this then the patient is considered definitely dead and gets recoded cat3 where someone would be sent ideally within the hour to formally recognise this. Any other time all cardiac arrests are treated by the call takers/dispatchers the same - be it a 13 year old or a 83 year old. If it's busy/location dependent there sometimes just isn't the resources to send but they'll send what they can.
Not everyone in greens on an ambulance is a paramedic. It can be that the first on scene is actually a hospital transfer crew for example so you'll have two people who look like paramedics, have a defibrillator and know how to use it and be trained very well in CPR, but not paramedics.
Initially whomever arrives first establishes that the patient is indeed in cardiac arrest (not just unconscious etc), and that there are no circumstances which point to the patient clearly being deceased i.e. they have rigor mortis because they died some time ago or the arrest was unwitnessed and it's likely no cpr has occurred for 15 minutes and the patient has no electrical activity in their heart. Assuming these conditions are not met then CPR is initiated by whomever arrives.
CPR - initially pads on chest, shock or not shock, chest compressions and starting to breath for the patient. You then work in 2 minute cycles with chest compressions and ventilations before reassessing. If just two people on scene or no paramedics you might not move beyond this type of CPR. When more people/paramedic arrives you try to progress to what's known as advanced life support, the main difference is that some drugs can be administered, but it's the 'basic' bit that's most important.
As a paramedic what you're hoping to find is that the cardiac arrest was witnessed and recognised, good quality bystander CPR was started straight away (any more than 4 minutes delay and even the young 'healthy' patients have a significantly diminished chance of recovering), amazing if a community defibrillator was available and used prior to our arrival, that the patient is in a shockable rhythm - if all this is present you're thinking this is one of the patients you might be able to get to hospital with a heart beat. Otherwise you're working on them to try and get them into a shockable rhythm. Generally speaking, if this hasn't occurred in 30 minutes we stop efforts to resuscitate at this point (it used to be 20minutes but evidence suggested increasing to 30 a year or so ago).
Once resuscitation is ongoing, and there's enough people there to free someone up to speak with the family we'll try to make this happen, that person would explain what we're doing, then establish the events which immediately led upto the cardiac arrest and gather significant medical history for two main reasons - can we establish a likely cause, is there anything that might suggest that efforts will be futile. From this conversation, and events during resuscitation one of several events can occur:
Heart starts beating again - keep the patient still and let them stabilise for 10 minutes before carefully transporting to hospital.
May have met certain criteria to be transported with CPR in progress - there are certain situations where if the heart doesn't restart in the first few rounds of CPR, i,e, 6minutes or so ideally they'll be rapidly transported to hospital with CPR in progress - this doesn't happen that often in my experience.
Decision to continue CPR so that a minimum of 30 minutes CPR has occurred - hoing that in this time disordered electrical activity appears which can be shocked and converted into order and the heart is restarted, but if at 30mins there's no electrical activity in the heart, to stop CPD. This is the normal outcome unfortunately - if the patient hasn't responded in the 30minutes then that's taken to mean that essentially they won't respond and that's the time to stop. This decision typically can't be made for under 18s by most paramedics in most situations - they are more likely to be transported with CPR in progress.
Finally a paramedic is trained to and can make futility decisions. Whether this is to cease resuscitation before 30minutes, or perhaps not even start (i.e. arrive and recognise the patient is clearly in terminal stages of disease so not begin CPR with/without a DNACPR decision previously). This decision must be clinically valid and takes in all the information - everything from how long for CPR to start, what heart rhythms have been seen, what medications did they take/previous surgeries/known to have any organ disease, age/normal activity levels abilities, physical health... The information the family give at this time is only a part of the picture that leads to the decision. Family wishes do weigh on the decision, but they certainly wouldn't result in a paramedic stopping cpr they thought could be successful. For me a big part of this conversation is bringing awareness to the family that resuscitation likely isn't going to be successful, we're likely to be stopping in the house and their loved one will have died, do they want to be in the room in these last moments/hand holding or similar. Recognising that the family/NOK are essentially patients that we will be looking after as well, nothing is going to make this situation ok, but sometimes little things can make a difference.
In the information you gave it may be they were attempting to restart her heart for 30minutes (or 20 before guidelines changed) and she didn't respond unfortunately and the conversation you had didn't actually factor into the decision at all but was just the paramedics process for understanding your level of awareness/thoughts so they can support you as best they can through what they knew was going to happen next. But also it sounds like your mother was a reasonable age, you say she was overweight and had some chronic health conditions, I've no idea the times for starting cpr/crew arrival but this could have all been part of decision making that it wasn't a survivable event, the information you gave would only be a part of this picture and wouldn't have been a strong influence if the clinician felt differently initially.