The first is the obvious one and will yield immediate results. Two units each night and one on repairs is what I would be aiming for.
From the experience of the GWR IET introduction, 5 will be very hard because the Scots may repeat their demand to take over NR Scotland entirely and have a substantial cash bung as well. (Grayling really flipped at that one!) However, the Scots might play nice if they think Burnham will give them NR Scotland anyway and they really don’t need all the units as they become due.
Indeed, ScotRail/Transport Scotland certainly wouldn't make any such negotiations easy and that's if they're willing to come to the negotiation table at all.
The main factor that ScR would be considering in any such negotiations would be if they think they'd be able to nurse the HSTs along until that time comes, however long that ends up being, without the fleet suffering a catastrophic and sustained drop in availabilty/reliablity during this time and there aren't enough 158s/170s available to fill in for the increased number of HST failures.
Part of me wonders whether or not it'd be worth ScR taking on any of the displaced TfW 158s as a short-term fix in such a scenerio if they feel they can't achieve this? This is all purely speculative/pie-in-the-sky thinking of course and that it would be yet more money the Scottish taxpayer would have to cough up for it, but there is fairly recent precedent for it, namely the 365s coming north in 2018 for about a year to fill the breach when the 385 testing/comissioning programme (oh look, Hitachi again) ran into multiple delays, the whole cab windscreen fisaco included.
You can fairly criticise Holyrood/Transport Scotland for many things and for making utterly cackhanded decisions (as a Scot, god knows I've laid into them more than once over the years) and you can fairly place the entire Inter7City HST saga in this category as it's been a cascading series of cock-ups more or less from the start. Even though the fleet's availabilty/reliabilty is much improved compared to the early days, it still ain't brilliant, nor are they getting any younger (corrosion in the Mk3s is a major issue iirc) and regardless of when the lease for the HSTs is up, the unions understandably want them gone asap.
But I'd also point out the obvious that the hole EMR now finds itself in with the 222s/810s especially has been years in the making, was entirely predictable and is primarily down to a near total lack of cohesive management, the ever-present and borderline supernatural incompetence and chronic short term-ism of the DfT with the eternal
tight-fistedness fiscal conservatism of the Treasury backing them up. EMR ain't a company I'd want to be working in a public-facing or maintenance role for right now; to call it a dumpster fire would be the understatement of the decade.