Underhill was capped in the summer between leaving Ospreys and joining Bath (which I'd expect to see happen with Mercer, if he's in England's plans)
Got a feeling Shields was some f*ckery - Super Rugby season overlapped internationals so he played for England then went back to Hurricanes, then went to Wasps a month or two later
I think that capping Mercer while still playing in France would open too much of a can of worms tbh,
You could end up with a situation where you get players signing a deal abroad (i.e. with a Japanese side - especially as their season is so short) but also signing a deal to go straight to a Prem club after and saying they should still be eligible etc.