That adapter you've bought doesn't meet any safety standard, please don't attach anything to it that is supposed to hold your child in an accident. The middle seat also isn't structurally designed to take an isofix bracket. I'm astonished that a Hyundai salesman would recommend this, it's absolutely crazy and sounds like a lawsuit waiting to happen - hopefully not, as you're not going to use it, but potentially if he's advising multiple people that this is OK. Speak to Hyundai customer services and ask for their engineering or safety department's take on this, but I bet they would be absolutely horrified.
I also don't really understand how he's cut it - it looks like the brackets go right to the edges of the adapter, so how has it been cut? If he's cut a section out of the middle, then it will be totally useless anyway because they won't be the right distance apart so it won't be compatible with any isofix child seats. If he's cut the edges, then the brackets are thinner so how are they supposed to be strong enough to hold the child seat in a crash?
Before you sell the car, make sure you take it out so nobody else uses it. It's a danger to life.
Isofix is not necessary, anyway. Isofix is supposed to provide a plug and play easy installation option, it doesn't have a safety benefit in and of itself except for making installation easier so that errors are less common.
The type of bracket in your picture as on the Britax seat is not true isofix, Britax call it Softlatch, some other brands call it things like Isoclick or Isosafe or Isofit. It is similar in design to how LATCH works in America - still attaches to ISOfix points but in European cars, these brackets are not allowed to be the sole method of installation. So Britax have these connectors on some of their high backed booster seats, for example, where the seat (and child) are secured by the seatbelt. In this case, the latch connector is just to prevent the seat from sliding around the car as the child climbs in.
I agree that you would be best off going to a local car seat specialist. Not Halfords, their training is totally minimal - the in car safety centre is a good suggestion or if you can give a rough geographical area, I can look to see what else is near you.
Nania and Minnie Mouse (which are all team tex anyway) are not great brands but they are at least certified to safety standards which your isofix installation is not. Likewise some people will do 
at the Multimac, but Multimac is a tested safe option. If you have two kids over four and will have a newborn, this is not a terrible solution because your older two can go into the forward facing, harnessed seats and the baby can go into the Minimac baby seat until 13kg and they can fit into this for a fair while - I have seen pictures of 2 year olds in it (and one small 3 year old). It's best not to rush children into forward facing if you can help it. Once the baby is outgrowing the baby seat, then you can reevaluate whether you want to keep the multimac and have them all forward facing or get rid of the multimac at that point to put the baby in a rear facing seat, perhaps if one child is old enough to go without a booster by then.
In terms of getting 3 seats across your usual best solution is just lots of trial and error. Cosatto (assuming an all-stages?) and Recaro (assuming a booster seat?) aren't very narrow seats. The i-size booster seats are pretty slimline. Maxi Cosi tend to be narrow. Baby carry type seats are all typically around the same width, because of many of them having the same pram compatability. I think if I were you, I'd look at a solution which has two high backed boosters side by side on the side where the two buckles are facing each other, and then a belt fitted seat for the baby on the other side of this. This is so the older kids don't unclip the baby's seat by mistake. You can get some baby seats which have a belt fitted base, so you don't need to reinstall every time.
In terms of isofix vs seatbelt, it's normally easier to fit three seats across if you avoid the isofix entirely and go for three belt fitted seats. You just get a bit more flexibility this way. Ultimately, we needed to fit 2x car seats + a teenager and we decided not to go for Hyundai - we have a Peugeot 5008. With this we can use isofix seats, no problem. When we had a Peugeot 3008, we needed to use one belt fitted seat to get them all comfortably in at once.