Lots to unpack in your post, here we go:
It's OK to finish A-Levels when you're 20 and to be 21 when going to uni. Yes, most people finish A-Levels and go to uni at age 18 but obviously there was a reason why that wasn't the case for you and that's completely fine. What's important is that you're actually doing A-Levels and chasing your dreams. I'm so sorry your parents are being horrible when they should be being supportive
but don't let them bring you down Maybe try talking to them, tell them that you're doing this with or without them and so they can either support you or shut up.
You shouldn't drop RS because you will need to have sat 3 A-Levels to go to uni! You might change it if you think you'll like another A-Level more though! Practice and studying RS will probably make you increase your marks over time too. Edit: just realised you're taking 4 A-Levels total. Absolutely drop one if you don't fancy doing it. 4 A-Levels will only bring you more workload and no advantage whatsoever.
Mark schemes... they can be incredibly pedantic sometimes (I remember once losing a mark for using a word not in the specifications. The name was correct, but not what the exam board wanted so puff, one mark gone). But it's a case of practicing questions and looking at the mark schemes and you'll eventually get the hang of them
It seems like you have a plan to do it all! Doing GCSE double science and A-Level Biology will help you a little in the sense that your A-Level Biology will effectively cover GCSE stuff and then some more, and then the Chemistry bit of double-science will help you on the biochemistry side of Biology. They'll help each other, in a way. Your first year revising both A-Levels and GCSE content will still be a bit of a challenge, but I think doable - hopefully somebody who has done what you suggest will reply and confirm it.
You're not old! Honestly, nobody will tell or care about your age at university and you'll see that when you're a "young adult", age is but an illusion
Again, sorry about your parents, but don't let them get to you. You're doing what you want to do and that's mighty fine of you, don't let anybody stop you.
As I previously said, you need 3 A-Levels for university so you cannot drop RS and do 2 A-Levels. You can change it if you wish! See the entry requirements for diagnostic radiography to see if any university has a particular requirement for specific A-Levels, but I think they only want a science or two and you can do what you wish for the third one - but please check online on each uni's course website.
Don't do AS exams. Universities truly don't care about them, just the real A-Levels. That's why it's increasingly uncommon to do AS now - they're to all effects and purposes absolutely useless.
Wish you the best!