I think part of the reasoning is that the 2 Southern services are only about quarter of an hour apart at Fareham in the westbound direction, so if the train from Vic is 15 min late it is also delaying the train from Brighton, and thence delaying the SWT stopper. So if they cut the Victoria short passengers should be able to have a short wait for the following SN service.
AFAICS the current fast trains (SN/GWR/SN) run as a flight of three from Fareham towards Southampton, so the obvious place to put an extra SWT fast service is ahead of the first SN service by the 6 min headway? But that means it probably would leave Portsmouth just before the next SWT Waterloo via Eastleigh service. That then asks the question, where should it call between Portsmouth and Fareham?
The other point about diverted Weymouth or Poole services, is what priority does that spare path take in the overall scheme of things. Perhaps it can just be fitted in as well as you suggest, but it would have to arrive into the mix at Cosham at just the right time...