The thing about the frigates is that they'll expect a share of the UK forces, & since they can't have the big ships (you can't give them 20% of an LPD), they'd have to get a bigger share of the rest. That's logical, as they have a big EEZ. Otherwise, they could demand cash compensation, & I don't see that being agreed to.
If they don't get any frigates, what do they get? All the OPVs? Still less than their share of the navy, & we'd have to build more OPVs to make up for those we've handed over.
Denmark & Norway (similar populations, lossa fish-filled waters, offshore energy, particularly Norway) have frigates/destroyers, & intend to keep them. Both have built or are building new ones. They also both have much bigger patrol fleets (look at the Norwegian Coast Guard!) than Scotland can inherit from the RN.
I take your point but we could always just say "Ocean, here, take her away...or Lusty if you prefer?" Ocean is almost reasonable but not very useful for the Scots and Lusty would be a poison chalice given her crew requirements.
Scotland really would want OPV's as they're cheap to run - it'd be dirt cheap for the UK to just buy out the commitment by some adjustment in cash to the order of a couple of hundred million to provide for a fleet of new build OPV's - instant political success for Salmond as it's "new jobs for Scotland" and win for the UK as we'd not be letting go of surface combatants we still need but Scotland can't realistically crew.
The Navy element is the worst, most complex conundrum to resolve - the army has plenty of spare heavyweight gear in mothballs to hand over, thanks to recent cuts, the RAF can let go of Tiffy Tranche 1 - but the RN has a job to do that doesn't get less stretched if we let go of Scotland.
Scotland realistically doesn't want to spend a lot of cash on defence - the SNP is basically a left wing, socialist undertaking, nationalistic and inward looking. The less they spend on defence, the happier they get in terms of balance of payments.
It'll be like most divorces, difficult and unsatisfactory in terms of who gets which car, but possible.
Ian