Snails could work, I suggest a Zebra Nerite snail as they are small. You can put in one or two.
Again, a tank with larger bottom space is better, but isn't as crucial for snails as they also climb about the walls.
Keep in mind that snails create a
lot of waste, so be sure to clean the tank often.
Even snails need a cycled tank, so just explain to your little brother that the fish will be very happy and live longer if things are done right.
Live plants will help to reduce ammonia levels a little bit, and nitrate levels. They are quite a lot more work than fake plants, as they require special lighting and substrate. If you feel ready for the extra work, then yes a live plant would be perfectly fine.
Another option is to get a small bamboo stalk. They are the easiest to care for, in my opinion. I have one in my 20 gallon tank and I have used gravel and sand as a substrate, both worked fine. I don't have special lighting, just incandescent.
Be sure to keep the top leaves of the plant out of water, or it will start to rot. Mine survived a couple months completely submerged, but unless you want to keep buying a new plant, be sure to keep the top out of water.
'Most' snails are tropical, and need temperatures around 75˚-78˚F. So, yes you will need a heater. If you don't want one, then there are some snail species that can live in goldfish ponds, which are cold water fish, but can't think of the name of the top of my head.
Yes they will need a filter, as mentioned above they are quite messy.
Good luck! Hope I could help
Don't forget to watch out for your brother and make sure he doesn't dump in a can of fish food again.
P.S Another idea which is the best would be to hold off on fish for now until you can get a 4 or 5 gallon tank, which will open up more options, such as African Dwarf Frogs, or another Betta.
A 10g would be good for a nice tropical community set up, but may be a bit much to handle.