Okay.....4-5g....IMO-I wouldn't add any fish...but maybe 3 dwarf cory cats...but that could be risky in 4-5g and they really do better in groups of 6 or more
Maybe 3 otto, once the tank has matured some in about 4-5 months-they would work
snails, different shrimp like red cherry or the yellows
Not a lot of swimming room and with limited swimming room come stress and more chances of the Betta catching them even with hiding places and plants...check into kuli loaches-I know they need to be in groups, I have never kept them but they are small and burrow I think....
He should...however, if he is working on the one in the back that is harder for you to see, he may be doing this for a reason...some fish don't like to be watched and can feel safer and do a better job, so you may want to leave it to help increase you chance for success...now that I think about it......
Fish will often eat their eggs/fry when they feel threatened-as odd as this sounds they would rather get the nutrition than let something else get the nutrition-they do this so that they will have more energy to spawn again in a safer place to improve chance of success.
Oh and... one more question, if you don't mind-
there are two cups in his enclosure. He's working on the side in one and he's working on the middle in the other. Should I keep them both in there or remove one?