You're going to run into the mounting problem no matter which computer you choose. The most common solution is to zip-tie the magnet on the small chainring instead of the crank arm. You should be able to figure out how to wrap the zip-tie around a chainring bolt while keeping the magnet out of the chain path. Then, mount the sensor to the derailleur post. You should have enough swivel range to make the sensor work.

I've never yet met a computer wiring harness that couldn't be lengthened by splicing in some 18 gauge wire. Some are easier than others, though...