Well I mean I consider single player to be similar to single player in SCBW. Where you can use cheats and stuff vs comps to either test builds or just have fun. As far as I'm concerned if this strategy breaks down in the face of an additional player, then the situation is limited and under control.
Maybe this works for you but I would very much enjoy having a 'fixed' single player mode as opposed to a 'broken' one. Particularly since the only competitions we've had so far have been based around single player. For now I'm just staying away from double interest, playing multiplayer and random.
Alternate suggestion: Single player interest period should be fixed to 20 seconds; however, subtract 1 second for each additional player. That would make 7 player interest compound in 14 and 8 player compound in 13. Could be an interesting experiment to increase the impact of interest on these games, where it's currently almost non-existent.
i dont think single player is broken, as this is not a single player game and if your looking to exploit it you'll find a way. Personally when i was playing and the other people in my game died i got to level 50 or so with 50 lives. i killed all but one creep,sold all my towers and let him run through the maze until i had 10 lives left. by the time he did that i more than quadrupled my total cash with little to no interest upgrades, i was playing on random.