Can the Lizzie Line accept whole 400m train’s worth of passengers?
Yes. A whole train's worth can easily be absorbed. Class 345s hold more people than a 400m HS2 train (albeit via most of the capacity coming from standees) and Paddington terminators will be extended to OOC. The issue of capacity at OOC is
how many 400m trains' worth of passengers can be absorbed by the Liz.
You'd also get inbound GWML passengers transferring there (easier than at Paddington).
And there's the problem of peak crowding: Reading trains will be squashed standing room only from Slough, Heathrow trains the same from Southall. OOC terminators are the only trains passengers heading into London in the am would be able to board.
Phase 1's full service is between 10-13 200m units an hour (depending on whether Birmingham trains double up)* - with phase 2 being more than double that 27-28 units per hour and there's only 12tph Lilac Liz Line due to terminate at OOC at peak times (10 off-peak).
Some would go West, some might get a GWR to Paddington, but most are going to get on the Lizzie as there are no other proper connections (it really needs a people mover to Willesden Junction and more platforms there....)
Indeed - hence why Euston is needed ASAP. It can cope with a slight delay and rethink and that to improve the , but they need to be rebuilding it again before the year is out - maybe next year if the rest of the line is running later.
The Liz is a single point of failure for onward travel, even if it copes with phase 1 traffic, it's really not a good idea to have it beyond a short initial period of maybe a year.
*this, of course, assumes there's room at 6 OOC platforms for 10tph to terminate. It's doable, especially as Euston is planned to be fairly quick turn around with the longer layovers at the north ends, but it's tight. Euston is probably required just to run the full phase 1 service.