Applying paint in hard to reach regions due to roofing sheets
I think the answer to your first question is already evident to you. If you cannot get to it, you cannot paint it. They do manufacture some specialty application tools for tight areas, such as mini-rollers and "paint gloves":



My experience with these items has been less than fantastic, but maybe getting some paint on those hidden areas is more important than aesthetics.

As for your second question, NO you do not need to remove all the existing paint. You need to remove all the existing paint that is not sound (i.e. likely to peel or flake off) and de-gloss all the remaining areas. This is normally accomplished by mechanical abrasion (sandpaper, steel wool, etc.), unsound paint will come off when subjected to sanding and the rest will be de-glossed. You should thoroughly clean after sanding, and apply a metal primer on bare areas prior to repainting.