I think the cristal tower was made the right way. For future coil turns or content lockout, SE should just lock reward once you complete the content.

Let's say someone completed T6 & T7 on Monday. They have lotted (or not) and got a few of the new tombstones. Then a friend of theirs come and want to run T6, but is 1 tank short. They should be allowed to come again in T6, though they won't be able to lot (pass only, wether they obtained a lot or not in their 1st run of the week) and won't earn any tombstone.

This way the reward lockouts remain (which is the primary reason why we can't do coil more than once a week) but people will still be able to help each other to progress