1) Is the mender a part of the actual raid and if so do they take a slot? From your last statement it doesn't appear so.
2) Do you have to cast it while being part of the raid group or can you drop from the raid first then cast it? I.e. essentially leaving it in the zone, but not in the raid.
1. You invite the tinkerer into the raid.
2. The tinkerer summons the mender bot while in the raid.
3. Tinkerer /exit's out of the game thus leaving the raid.
4. Mender Bot stays in the zone.
5. Invite a replacement back into the raid that was waiting outside the zone or whatever.
6. Enjoy repairing the whole raid.
I hope that clarifies any questions anyone may have.