I am not a lawyer so take anything I say with a grain of salt. This is based purely on my own experience and what I have seen.
Usually in terms of sharing hacks people will share patches which are then applied to the ROMs, since you are not sharing the ROM itself (which would be illegal) but rather the modifications you made to it.
Obviously selling it in any way would not be allowed as you would be making a profit of of it. Making a profit is not the end all be all in fair use/copyright cases but it is taken into account.
There were two main cases that I know of of Nintendo stepping in and taking down a ROM hack: Pokemon Prism and Pokémon Uranium. Both of these had a few things in common.
1. They were both pretty popular and got a lot of attention
2. Both of them (if I recall correctly) were hosted on their own websites, thus making it more difficult to differentiate from an official Nintendo game.
In my opinion I think it was moreso the latter reason why they felt the need to step in.
Think of it like this:
It's one thing if you are posting your fan fiction to a dedicated website for fan fiction (such as FanFiction.net) where it is made pretty clear that the works there are not the copyright of the writer and are instead written purely because the writer enjoys the source material.
It may be different if you present your fan fiction in a way that's more in line with self-publishing a book, because in that case it may be more difficult to tell whether or not your work is an official work or not/whether you are affiliated with the creator or not.
(Whether the work is in good faith is also sometimes taken into account in these sorts of things too. Most rom hacks are in good faith as they are not intended to steal Nintendo's (or whoever else's) IP).
So if you share your hack in a forum like Pokecommunity or some other forum where it's pretty clear that it's a fan made game, and you don't get money involved, you should hopefully be free from any take down notices
(I also want to note that nobody has been arrested for making a Pokémon ROM hack. I think the worst that would happen for this sort of thing would be having to pay some amount to Nintendo (if it even went to court, which it has not. Not even in the cases of recorded take down notices as those simply said to remove the download links. The authors complied and nothing more happened.))
As for your other question, Pokemon ROMs are ESA protected, and downloading them would be illegal (which is why there is a rule on Pokecommunity against linking to ROMs or ROM downloading places).
I've looked into this a little though, and there seems to be very little on ripping the ROM from a physical copy that you own. (for GBA Pokemon games anyway. For NDS games, in the game manuals it specifies that even if you rip it yourself, that is not allowed. But those are two different systems and may have different rules. Also this manual is ten years old so there may be different rules now.)
TL;DR Distributing and downloading ROMs is technically illegal. Ripping it yourself may be fine for GBA Pokemon. Jury's still out.
When it comes to sharing your hack, stick to defined hacking forums and use patches. This makes it easier to know that your game is not an official Nintendo game (making it less likely they will feel the need to take your game down).
The worst that could probably happen is Nintendo politely asks you to take down your download links. You won't be arrested and likely will never have to go to court unless you don't listen when they ask politely.
Hopefully that answers your questions!
