When I changed around some ADRs after making my FPs, I believe it warned me about the times overlapping a bit, but it didn't stop me from booking. I guess they do that to give you a heads up so you can't really complain you didn't know or weren't told.
OP, if you want to book something close to a FP, I would go for 10:45 instead of 10:30. Even once in the FP line, there's no guarantee there won't be an additional wait inside. Lots of times we've moved through the line very quickly - to the point where this past NYE we literally walked on to 7DMT despite an insane 150m I think standby line - but lots of times, especially at BTMRR, we have encountered an additional 10-15 minute wait once inside the queue for FP. Once on it, that ride is on the shorter side, so once you get through the queue, barring any mechanical issue, you will probably be off pretty quickly.