Yes you'd be able to see it - but you wouldn't be able to tell if it was faulty or not, as it's the internal impeller that goes.
Probably not that difficult to change, so long as you can get access to it. I think you'd need to lock the crank & camshaft in position first though, and you'd need to have access to release the cambelt tensioner.
I wouldn't rule the waterpump out - the fact it only overheated on a motorway run but behaves more OK on easier, lower load driving does sound similar to when mine failed.
The fact that it's slowly getting worse could just mean the impeller is starting to spin more freely on the shaft, so you're getting less and less coolant flow.
