Argh sorry but so many myths flying around on this thread. I will try not to get sucked in but just to correct a few of them.
- 5 point harness car seats ARE escapable even if they are fitted correctly. Children can suck in their tummy and this creates a gap that they can use to get their arms through. This is the case in all 5 point harness seats. You may find a disclaimer in your manual about it. It is easier for them to escape if the straps are too loose or set at the wrong position, but it is always possible to escape. Not all children realise they can do this. Escaping does not mean the seat is wrongly fitted. (But, yes, you should check whether it is!)
- Chest clips are not actually illegal. Under the old car seat regulation R44 it was true that the car seat had to be opened in one motion. However most car seats for sale today conform to R129 which does allow the use of an extra chest clip. I think three or four seats have one that I know of. Most do not. It's not illegal to add a third party product to your car seat, though most manufacturers won't recommend doing this.
- Add on products for preventing escape aren't necessarily unsafe. Some caution should be taken - the biggest no-no is if you're using a third party product to mask a poor harness fit, as a poor harness fit is dangerous. IF your child is escaping due to a poorly fitted harness (again, the harness does not need to be fitted poorly for escape to be possible, but it IS a common cause) then fix the harness first. If the product isn't masking a poor fit and it's not changing the way the harness moves, then it's not necessarily unsafe. Properly restrained is better than them being unrestrained or restrained only at the hips - this is very dangerous especially if forward facing. I do agree that extreme measures such as padlocks and tape (and a buttoned up shirt) are a problem in terms of removing the child quickly from the seat in emergency and this should be considered.
- Nothing like this is going to "invalidate insurance" that is not a thing.
- Cars were not safer years ago when car seats weren't required. Many more children died in car accidents. Children dying in car accidents today is very rare, because most people use car seats and modern car seats largely work very well and are less easy to use incorrectly. Most children who die in car accidents today are either in catastrophic crashes with adult fatalities or they are unrestrained. Car seats are not required today because of some kind of nanny state or because driving today is somehow wildly more dangerous (it's a lot safer) it's because things generally improve and get better over time and that includes safety standards.
General tips:
Car seat harness straps move up and down to accommodate child growth. You should ensure they are at the correct height. Your car seat's manual overrides random internet lady, but in general: Correct harness fitting is that the shoulder straps are roughly level with the height of the child's shoulders, if they move with a headrest, move the headrest so the child's head is within it and shoulders are just below. If child is between two settings, choose the lower for rear facing or higher for forward facing. Straighten out any twists in the straps. They can prevent the harness tightening properly, cause wear on the straps and cause pressure points in an accident.
Shoulder straps must be close to the neck. If the straps are tightening around the arms or shoulder tips in a V shape, the straps are probably set too low. If you have pads on your straps and they are curving up and over the shoulder, the straps are too low. They should start on top of the shoulder and curve down their chest (like a lower case r).
If child is too tall for their shoulders to fit under the top strap position, the seat is too small. Try Joie Bold for forward facing or Axkid Move for rear facing. (Unlikely at 2 but possible at 3 or 4)
Avoid bulky winter clothing in the car, it makes the straps too loose. Correct tightness is that they should be snug to the body with enough space to fit one finger between at the collarbone. Too tight can be uncomfortable. Most people make them too loose and can tighten further, but if you've followed this advice and overcorrected, you can relax it a little bit!
If you're going to add a product to the seat, the safest option is one that's branded to your seat. Besafe and Cybex sell clips which are crash tested with their seats and that they approve. They are however expensive. If your seat is another brand, 5pointplus (which is the link the OP originally said she had ordered) is good, this is a fabric cover that blocks the gap they can reach by sucking in their tummy. It doesn't affect the single release, crash testing on it is also good. Some Cosatto seats have this built in, but you don't need a Cosatto seat to use it, you can use it on any brand seat. For chest clips, the best is considered to be Besafe Belt Collector. People on car seat groups are very anti any other type of clip - they are probably not that bad in reality. But if you are buying one it makes sense to go for Besafe Belt Collector, because it's made by one of the manufacturers that is right at the front of safety innovation and research, rather than some random seller on amazon. There is a theoretical risk that a hard plastic clip could injure the breastbone or place strain on the harness whereas the Besafe clip is flexible plastic similar to American chest clips and is designed to fail safely in an accident.
Safest type of car seat is rear facing.
Second safest is good quality, highly rated forward facing, 5 point harness seat being used 100% perfectly according to instructions.
Third safest probably forward facing impact shield.
Fourth likely forward facing seat with minor misuse and/or of a more basic style.
Unrestrained/improperly restrained/major misuse is very unsafe and not legal.
So, if you can get your RF or good quality FF seat used properly with the use of a third party anti escape product then it is worth doing. If this is not possible, then an impact shield seat is a MUCH better option than an unrestrained or partially restrained child and may even be safer than a more basic FF seat.