Some of them are odd conventions.
It is polite to offer the last of something to everyone else before you take it.
Equally, it would be the height of rudeness if someone IS offering you the last of something, that they clearly want, to actually take it!
If you are sat at the table being served its polite to wait til everyone has their food - however if you are the host serving, it is polite to tell people to start without you rather than let food go cold.
Much of it is about all sitting down together in a relatively small space, and some is about being served by others.
Knife and fork open at 5 and 7 on the plate = I am still eating, don't clear my plate.
Knife and fork together neatly on the plate = I am finished, you may clear my plate.
This makes bog all difference at home with family, but if you were sat at a big table with wait staff clearing and serving, it is the difference between having your plate cleared or a member of staff having to politely cough and interrupt conversation and ask if you if you're finished.
Similarly, elbows on tables causes you to lean forward. Sat at a long straight table, you're then stopping anyone next to you seeing/conversing with anyone the other side of you (and whilst conversation across a person is also rude, blocking them from having the option of saying 'would you pass the salt' or being able to react to something terribly witty they just said is pretty rude).
Sat at a round dining table or a table with fewer people at it, this is no longer a big problem.
I think people latch onto some of these things as ridiculous without understanding their context.
They also forget that it is the pinnacle of bad manners to draw anyones attention or comment in any obvious way, to any etiquette faux pas! The British way is to say nothing at all, the judge the hell out of them later in your head! Obviously! (Joking, for the hard-of-humour).