with loosing front mech and shifting by hand, maybe a dumb question, how do you avoid potential chain drop? i know with a single chainring you can use a chain guide/guard to stop chain slipping off, that what i had on the bike originally. But would that also work ok with a double chainring? the guide/guard would need to be bigger to allow room to change over but that might then allow the chain to drop into lower chainring due to side waving?
Using your existing crank arms would make it difficult to use a chain guard, although if you used extra long bolts and some chain ring spacers you might make it work. Unless your chain is currently rubbing on the chain rings there doesn't seem to be an issue with the chain dropping from the big chain ring to the little. I would just remove the FD and see if it works with out a chain guard. The original set up had the big ring on the inside of the crank arm spider and the chain guard on the outside, so there may be some chain line issues but hopefully not.