Thank you!
I do try. I am not a fan of the FB groups - too echo chambery and judgemental. It's a shame because I did learn a lot there and I do find it fun to geek out over car seats (yes perhaps I should join one of the various Dull groups instead
) But to be perfectly honest, it is very difficult to tell fact from fiction on them because myths get repeated so often people believe them without question, and it becomes extremely difficult to challenge it because the format of FB groups does not encourage nuance. When it got to the point I was literally seeing the myths grow from nothing to solid, unquestioned, gospel truth I couldn't take it any more and unfollowed them all.
I have not managed to find any relevant evidence supporting either the bone ossification theory, any specific age for turning from RF to FF nor the idea that FF harnesses are worse than seatbelts.
It is definitely true that RF reduces risk compared to FF, and the younger the child the more extreme the difference. If you can make it work then absolutely do because it's worth it. If you can't make it work, get the best FF seat you can. If you can't change the seat you've got, read the instructions and follow them to the letter. Read them again because you'll have forgotten things. (I have.)
You can RF in a small car. (But not with every seat)
You can RF on a budget (But it's not worth going into debt or financial hardship for)
There are RF seats designed for giant tall Swedish children - they have leg room built in. I can put up a picture if you like 
It literally doesn't matter which RF seat you choose. It might matter which FF seat you choose - there are more differences between them and the differences matter more.
Using the seat properly (or at all) is much more important than RF/FF. Most children killed or injured in car crashes are unrestrained or improperly restrained. If you're using a good quality FF seat properly, you're already doing better than average. If you find out you've been doing something wrong, don't feel bad, be glad you found out! It's usually easily fixed, and no harm done.
If the law was changed to mandate RF until age 4, it would save about six lives a year (UK figures). I am not saying that those six children aren't important. Of course they are. But it's really not the major issue that ERF advocates make it out to be. The law (15 months) is adequate IMO but I do want parents to know that it is worth RF longer than 15 months if it's working for you. Personally I would aim for a minimum of 2, preferably longer. But any increase in RF is worth it.
I don't think it's helpful that people seeking advice about FF seats are chased away or only offered impossible perfect solutions, and I extremely dislike the words "safe" and "unsafe" used in this context because they are totally misleading. You can only make relative judgements about this, not absolute. Safer/less safe. It's fine to have a personal line that you don't feel comfortable with - I don't think it's fair to impose that line on others. Everyone makes their own risk assessment.