4) Is it a good practice to wash your body with degradable soap in a stream/lake?
NO!!! It is not good practice, hopefully this question is as near as you have ever come to doing that, read the soap label, it tells you not to do that. Trail head instructions also frequently tell you not to do that. Do NOT treat wild streams, and, worse, lakes, as if they were your local sewer system. The stuff degrades, yes, but slowly, over time, not instantly. That means if you put it on the ground, it will eventually biodegrade into the surrounding soil, over time, that's all it means, and it should never be taken to mean anything else. So that is what is known as a 'bad practice', one of the worst, it's disgusting seeing that type of soap scum floating down a pristine mountain stream, polluting it for all other users downstream. You don't need soap to get clean anyway, you can just use sand to scrub yourself off, that works really well, try leaving some of the ideas of what you 'need' at home, it's educational, you don't need much that you think you do.
That of course also goes double for washing pots etc, no soap in water. Sand/dirt works very well to wash off pots, as long as you avoid too many animal fats, which are very hard to get off without hot water and some soap. Dry dirt will lift off fats decently too, then you just rinse it out. I know it seems counter-intuitive to clean with dirt or sand, but it works really well, in my opinion, much better than soaps for cleanup of cooking stuff.
It's easy to wash using a pot as a pouring thing away from the stream, or a dirty water bag for a sawyer filter if you use one of those.
Depends on the fruit, orange peel basically for all practical purposes does not degrade, and isn't natural to most environments you would ever hike in, I think it varies fruit to fruit.
Of course the real question re bananas is, how on earth do you plan on carrying a banana on a hike and not having it turn into brown mush by the end of the day, unless I guess it's in a bear cannister? That's quite a challenge. However, dried banana chips are pretty good and have no skins, and also weigh much less. Apples are good and are somewhat natural to most areas around the world, or are now anyway, and they degrade really fast, that is, if a mouse doesn't find it first and celebrate its great luck.