1. There is nothing wrong with sourcing from different suppliers to sell in different markets. In fact, there is nothing wrong with sourcing from different suppliers to sell into the same market. If Drop-Shipping, the thing to watch out for is to try and keep your shipping times and any warranty inclusions as uniform as possible so as to save any confusion from customers.
2. Yes, if you are listing on the same platform (eBay for example), but in different countries, you will be charged for each countries listings. One listing fee does NOT cover all countries, it only covers the country you are listing in.
3. Sure, you can list in as many categories as you wish, providing you don't have any restrictions set by the selling platform for being a new seller or similar.
4. Now, this is something you really need to discuss with each supplier that you deal with.
5. I'm sure you could use a simple mail forwarding company for this type of thing, but be aware that it will add quite a lot of cost to you in doing so. You should really speak with your suppliers in regards to their warranty policies before worrying about this too much.