If you do it as a Request First exchange with Interval though and it has not been confirmed, you can take back your "home" week and change your MVC booking.
True. But that's not a 'deposit.' Edit: Trade power is locked-in at time of deposit. Waiting 6 months (?) will theoretically decrease the trade-power of the deposit. It's not clear, but the OP indicates the goal is deposit in II, not to actually use the unit. So, it's a question of what gives the highest deposit value, high-TDI MGV at 6-12 months or FC unit at 6 months? My vote is on 12 month high-TDI MGV. No guarantee of even getting the FC exchange.
In theory, you can search with anything. edit: You'd need to request first with MGV and then cancel once the FC exchange was made. So you could search for up to six months via request first, pay the cancellation fee (Marriott) and then cancel the II MGV exchange request and place the new request with your FC deposit. IF the FC exchange failed, then you'd deposit your (<6 months) MGV into interval and convert your request to a deposit first request.
But, I think the ultimate goal was to get the absolute
best II deposit available for the 2018 use year. So it's back to the chicken or the egg: Is the high-TDI EARLY MGV deposit better than the high-TDI LATE deposit for an FC unit? And, is it better to have a six month lead to search--via deposit first--with the MGV, or is it better to search for six months with MGV (request first) and then search via deposit first after booking the FC unit (if available) or finally just depositing the MGV; which is now late?
I think the priority is:
High TDI (they both are)
Early deposit (only the MGV deposited now is early)
Deposit first (vs request first) if you're not using the unit.
So, that would favor: depositing a high-TDI unit, as close to 12 months+ from reservation, deposited and search requested as soon as possible.
I don't think waiting six months hoping for an FC exchange and depositing a unit <6 months from check-in, searching via request-first in the mean-time is a 'better' solution. IMHO.