Do Hull Trains really need diversionary capability? Aren't Hull Trains and Lumo both First Group owned? If so, they don't appear to mind not having diversionary capability at Lumo, so presumably the diversionary capability at Hull Trains is just a helpful side effect to them having bi-modes anyway due to the last bit still waiting for wires.
LHCS already runs on the ECML in the form of LNER's IC225 sets (although they are sadly now confined to services south of Leeds/York, but I understand this was due to avoiding having to keep doing traction refreshers for Scottish crews rather than LHCS being slower - with a shorter rake they probably wouldn't even be noticably slower).
No, diversionary capability is a fundamental part of the Hull Trains business plan, always has been. It’s not a side effect and is quite deliberate, I can assure you of that. Hull doesn’t get Type 1 and 2 Schedule 4 money so it needs to divert when there is engineering work and it also helps when you have those all too frequent wires down incidents on the ECML. Being able to go via the GN/GE Joint line (or via Goole) is fairly fundamental.
Lumo was not developed by the same team at FG and has a different business model. If I had been involved, they would have had full diesel capability.
The class 91 situation is a world away from class whatever and mark v’s. If you can’t meet 22x/80x timings, you won’t easily run your services - you need to fit on the ECML, not get the ECML to fit around you. That was one of the reasons HT went for 80x - they needed to be able to match the LNER 80x units, especially with an ECML recast on the cards.
With interest rates starting to fall, it remains to be seen if Hitachi unit prices get competitive. We in the industry will shortly find out if they are starting to move towards acceptable levels.