They would not be very useful for fieldcraft. I know it sounds like a good plan, but those rifles are quite fragile and would break after a couple of outings. They are £35 for a reason. For rifle drill on the other hand, they work.
If you want to do fieldcraft on a parade night with rifles and have no DPs, why not make friends with your local ACF, go over there and get some joint training in.
I don’t see the problem with using the same weapons for both blank and live firing. What do you think everyone else does?