I'm pretty sure they do this so that you will spend money for donuts they do need for people to by and the amount required to fill out team is rather large so yes its their main moneymaker I'm betting for the moment minus some new suits.
This could be negative to their actual economy so I don't believe they will allow this, donuts trading can be abused to to an extent so yeah this probably wont go threw.
As to keeping enough donuts on hand I haven't had any donuts problems till i was over level 200 and some, so not sure what your doing but you must be spending them on something that you might not need or is not helpful enough no clue their. would just say only things I've ever spent donuts on was boss trying for last hit, last 2 energy refreshes when trying for the 15 mill achieve (wouldn't try for it unless you were at least 170 ish or higher though), missiles ( which i found out is not very helpful till later in game stupid me) and items in shop besides missiles that were at least 10 points better than current.
As for some reward/donatible donuts got to side with ATR something for achieves like that would be a good idea its less likely to be abused and is a nice reward for leveler's and their teams. Getting to level 100 takes awhile and getting to 200 takes way longer and 300... yeah these are very unlikely to be abused if they are the grounds for donut awards for you to be able to donate to the team.
Also in good faith like ATR pointed out make them equivalent to level basis like 10 for lvl 100 this makes them less abuse-able and will not likely harm the games economy very much if not at all.
Sadly I don't think this will be implemented but it does have promise Here's hoping ((^))
V