Not sure about the Scottish, but league rules state he can't play against parent club. I think its a bullshit rule (as does UEFA) but Motherwell signed up to them rules.
As for the second point. You can't seriously think Celtic would be in the wrong if that scenario happened?
Although it can't as his contract runs out this season so for Celtic to get any money from us they would have to offer him a contract and be accepted
EDIT. Contract runs out 2019 not 2018.