A few thoughts:
1) the rate of electrification and the routes being electrified or partially electrified initially concerned limits the number of DMUs that can be replaced in the short term hence 156 replacement is the limit in the short term, not point in getting delusional about 158 replacement any time soon.
2) Even complete 156 replacement will be a challenge in the short term as it will need several discontinuous electrification schemes and sub schemes completed to release the 170s and 158s currently used with multi-stage cascades and changes to 158/170 usage on existing routes where DMUs are retained.
3) Structurally the 158 and 170 should be good till at least 40 years
4) What is proposed is entirely realistic for the first stage on rolling stock replacement.
5) 23m for new fleet is the way to go with some cascades of existing EMUs especially as it provides more space for batteries than 20m.
Very informative post. Hope the Scottish government see it as I'm left, maybe wrongly with the impression that they assume units can do huge distances just on batteries. Clearly not the case and they perhaps need to think more seriously about whether they want the expense of installing and maintaining 25kv vz sections for for running and charging or whether they prefer a bi-mode solution which would use an awful lot less diesel than throwing sprinters and 170s around on these routes
The problem with BEMU range on battery isn't now the theoretical range (which is large) but often the recharging time which effectively needs to be under the wires and mostly while moving else you require long lay overs or very high charging rates and much more expensive batteries. Transport Scotland et al. north of the border understands this well, DfT less so as DFT always want to try BEMUs on locations with short lay overs...
Also you want to try to optimise battery usage to maximise battery life span e.g. slower charging and avoiding frequent deep cycling where possible, which is again best done with long periods of charging under the wires.
*** The minimum sensible distance of route wired for sensible BEMU operation is probably 35-40% depending on service pattern, gradients and layover times. ***
The plan for Borders is electrify southern end where a power supply is available far sooner than the northern end near Edinburgh which enables sufficient battery charging under the wires on the southern end and in Edinburgh. With the gap be electrified later which then enable the BEMU to be replace with and EMU and the BEMU to be cascaded else where to another recently electrified route.
The break even point for electrification vs other e.g. battery or hydrogen is through to be 2tph at 3car before local detail are considered (charging, unit diagrams (and spare units)).
Bi-mode are not a realistic decarbonisation option they are carbon reduction option for the short and medium term cases where there is (already) huge mileage under the wires with diesel units, which isn't the case with Scotrail.