Prosthetic leg problem

I've read and watched videos about how to sit properly (in that really short tables and so) and it seems (or at least that is my perception) that if I sit in certain ways without being told I can do so, it would be considered as impolite.
So I wear a prosthetic leg.

How am I suposed to sit down in such places?
Is it OK if I just sit with my legs straight? or if i cross only one leg?

Also, I would like to know about takin shoes off and if it is very necessary at I dont know, restaurants or so...

Unless it is a formal ceremony or event, you should sit however is comfortable.

Even if it is a formal occasion, it would not be rude to sit in a way that suits your leg.

Some restaurants only have tatami mat seating but most have both mat and table seating. Many restaurants have no mat/floor seating at all.

Same with shoes. Many Japanese style restaurants (izakaya, washoku) are "shoes-off" establishments. In that case, you do take off your shoes.
If taking off shoes is a problem, just go to a restaurant that doesn't require shoes be taken off.

I do not think you will find very inconvenienced with those two issues.

I think it also depends greatly if it is obvious that you have a prosthetic leg. If it is obvious, nobody will say anything.

Then again, with some prosthetic legs, you cannot tell when clothed, only when walking.

Then again again, being the reserved country that Japan is, I doubt anyone would say anything anyway.