There’s a danger we are going to confuse people here, possibly because a 28 day reserve block can (and usually does) straddle two calendar bidding months. Just rowing it all back a bit to a CliffsNotes version of the relevant bidline rule (JSS version).
A bid for a reserve block will not be accepted if there is any overlap between Leave/DFW days off and any of the 21 available days of the 28 day reserve block..so no you can’t reduce a block down to 14 available days by having a leave in the last week. If you bid for such it will fail.
You can do what want in the Fixed Days off, but as Dupre rightly says you need to acclimatised on the start of the available days.
You can however have up to have to 14 credited hours of preassigned duties embedded in the available days, such as combinations of Golden Days, SEP, Sims....more than 14 hours and a bid for reserve will fail.