Line capacity is not a simple concept I agree, especially for simpletons like me or let alone the many rail industry experts that aim to part the public from their money unnecessarily. The link you allude to is here
Control Period 5 plans
The line capacity on a line pair is set by the maximum number of trains per hour each way, the numbers of passenger carriages and seating count, balance between the number of fast slow trains, together with the passing point availability, the signalling system used, the occupancy of the train, driver and train logistics etc.
Fast train services can originate from further a field if train occupancy limits allow, which is what I was [implicitly] suggesting on the Chiltern Line. How the Aylesbury Branch Line is used to spread the commuter load once East-West Rail is finished would also be a factor.
I agree capacities are reaching their limit on all London commuter approaches, which is why HS2 doesn't solve the capacity problem for any of the non-Euston bound commuter lines into London.
After spending £50 billion on HS2, a lot more money will still be needed to cure capacity problems in and around the rest of London as well as commuter journeys into serveral other big cities.
In terms of using Crossrail to relieve Euston: the last time I went to a talk on HS2 there was not going to be the necessary platform capacity at Old Oak Common to allow the logistical manoeuvres you describe. However assuming HS2 goes ahead, and enough platform capacity is built at Old Oak Common before work is started on redeveloping Euston, we will then all ask ourselves; What justification remains for redeveloping Euston at great cost?
As Sir David Higgins has just realised (slower than most) - HS2 does not solve the main transport link problems in the North either: travelling quickly between the main northern cities to alleviate the congestion on the M56 and M62.