However a new timetable is going out to consultation
But if the end result is a similar level of service as per pre 2019, all 701's will still be required on services previously worked by 455/456/458 & 707.
On that basis my estimates for traffic 701's (those in service every day) are 56 from 60 x 10 car, and 27 from 30 x 5 car*, the remaining being the maintenance allocation.
* A majority working in pairs perhaps (SX), but with scope to break down to 2 x 5 car for engineering work as you correctly point out.
I don't think there was ever a plan to use 5car on their own in regular timetable.
I think there will be a small number in the base plan, for the very reason you mention (siding capacity), so to answer class701's question, my guess is that some of the first starts of the day, and possibly some of the last services each day may be a mix of 10 and 5 car.
If this turns out to be the case, the latter (last services) will be the one's that need to be carefully planned to avoid late night overcrowding post pub and theatre chuck out times.
The devil will be in the (diagram) detail vs known passenger loadings.