We will not manage to move by 31st October...I am not sure what to do, whether not to sit at all, or whether to sit and put us on a waiting list - waiting list is operated until end of Year 7. Or not to sit the 2nd stage and once we have moved, to sit in-year admission test.

Consider sitting for stage 2, and go on waiting list when moved in inner catchment.
Some LAs allow change of address upto early part of December. Check those rules for the LA where you are moving in.

I think if I risk moving in Dec, I might lose the possible offer of a state school where we live now...and if Tiffin is not offered, then we would be stuck...not sure if my reasoning is correct...

That is correct.

I don't know about the reasoning but you seem to be dashing all over the place with a multitude of plans- remember:

1) Tiffin is a state school, just like the school near where you are now.
2) Any sudden changes of address that you can't prove may cause you problems when it comes to allocation of places.
3) There are very good schools all over the place, just because some don't get such stellar A levels or whatever is not a reason to chase off to move to be near others which TBH should do better as they are highly selective.

I would think very hard before moving house on the off chance of getting a place at a selective school. I would think really, really, really hard before moving on the chance that I would get a place at a super selective school.

However, perhaps you are confident of a place?

I just thought if we do 2nd stage, we can still have the option for Round 2 of admissions, once I have secured nearer school. Then I can move if I know if I am high on a waiting list...