i don't like that he is rubbing the belly. Keep him in the hospital tank for observation. You right usually if they do that its either the ich or velvet , or internal parasites. What kind of plant it is , i didn't know that they eat the rot i don't think it good though. Not really experience with that but never read that any one said that their betta eats roots of the plant. Also i don't remember that you mentioned the bump ....Where is the bump?
With internal parasites fish sometimes will have distended, discolored (grayish)stomach , the poop will be white, clear, broken white wormy, stringy shape. Usually if fish has internal parasites the his healthy will diteriorate pretty fast , also clamped fins, stop eating.
External parasites like you said fish will scratch him self and also would have white spots on the body or looks like the body sprinkled with salt. So keep eye on him.
About anti parasite food. When you do next water change you can take all gravel out and observe his poo if it the way i described above then give him that food. Also you can try to soak his food in the fresh squeezed garlic which is also preventative method for internal parasites.
Hmm i am wondering why he is scratching the belly?
And also fish can bite the tail because something bothering him including ext parasites. So yes keep eye on him .
Sorry its 9 pages can you remind to me how long you have him?