Yeah, later Opel ECUs get locked down a bit (just ask @SilentShout about the Siemens Simtec70 series ECUs on the Z18XE). Sounds like a decent plan though.

The F17 box from the Astra can be used with your existing shafts I believe and should bolt in, but you will need to convert your clutch pedal for a hydraulic setup, or set the box up to use a clutch cable. I'd be more inclined to go hydraulic, which I believe you can use a JR or JS Vectra clutch master cylinder for. The F17 box is probably the strongest box available for a Family 1 engine, but the standard F13 box will handle a fair amount of power too.

As for the mounts, you use your existing mounts. There's two types of mounts in the Corsa and they depend on whether you have power steering or not. C14SE with power steering and C16XE/X16XE mounts are all the same.