Crews? - no problems, the merry go-round will see the CHF convert to the Merlin and 28/78 convert to CH47. Trade-in the Merlin QHIs early in the piece for a CH47 conversion and blend in some Odi guys.
Who'll build them? - My guess is Boeing; AW will be busy with Algerian orders, Wildcat and Merlin CSP. Depending on the cockpit/avionics selected there might be work for Fleetlands (ie ship F frames over and outfit them here).
More Harriers? - More is always merrier. But, the Chinook is not "very vulnerable" to ground fire as a substantial number of engagements has proven (and, indeed, the historical record shows RW to be far less vulnerable than people generally realise...). I'd like to see a GR4 or GR9 carry on flying after RPG and multiple HMG/SA hits. In addition we have AH - and in many ways it is a better escort than FW. If you're concerned about Red Air then a CAS escort isn't really much help and since we'll have Typhoon to do the OCA/escort job I don't see it as an issue.
Future Wars? - The Chinook has taken part in every campaign the UK has been involved in since 1982; more than certain FJ forces I could mention. Therefore, logically, we'll need them in the future, so this is a prudent purchase.
Enemy Air Defence? - As above, Typhoon to do OCA. The lack of dedicated SEAD has been an ongoing issue for years. No doubt F35 LO tech will help, and RW assets will operate NoE in a non-permissive RF environment.
CH47F? - Not as good as a Julius Chinook in several areas, better in others. A hybrid that combined the best of both would be formidable aircraft.