A great many perennials spread with an underground root system and bee balm is one. But it is extremely manageable. If it starts spreading wider than desired, just sever the the outlying growth with your garden spade and remove. Or plant elsewhere or share with friends and neighbors. Bee balm looks best if in a sizeable clump so I would not recommend sinking it in a pot.....unless the pot you use is large enough to allow for a decent spread. You don't want just one or two stems poking up but several dozen!!

Yes, lots of ideas. You can go with annuals or choose a perennial that's the right height and a complementary color/blooming time, maybe a couple different flowers so you have something blooming more of the time. Dianthus, Blanketflower, Daisies, Four O'Clocks, Helenium, etc.

do they fly? It could be white fly and they make that black soot.... there are lots of different kinds of whitefly.... I am fighting some new kind that has just came to Florida recently from South America. It destroys peppers and tomatoes. It was getting on my milkweed which I had in my rose gardens because I wanted the butterflies in the rose garden. Had to pull it out because the whiteflies got into that with the awful sooty mold.

there is a name that plant forum ... also.. many bee balm smell like earl gray tea when leaf is brushed or crushed.. or at least fairly reminiscent of the mint family it belongs to... ken