From the letter, part of the problem is the physical capacity at St Pancras,
When you distill down what's actually written in the letter, this seems to be the critical issue.
partly the staffing by UKBA and the French Border Police ,
The letter doesn't actually say there is a shortage of potential staff. Instead it seems to be making the point that they can be much more efficiently utilised at St Pancras (due to the higher throughput) than at the other stations. That's what prompted me to wonder if that could be changed, by deliberately funneling a larger number of passengers through those stations than was previously the case.
and partly the need to have full trains at high prices to effectively make good the Covid losses.
The letter doesn't say that either. They just need to generate as much revenue as they can, don't they? A train doesn't necessarily have to be full to make a profit. I don't see that anything has really changed since pre-Covid: any train that turns a profit is worth running. I don't imagine that they habitually ran loss-making services before - they weren't under any kind of minimum service obligation are they?
Obviously starting 900 seat trains from Ebbsfleet, which would have to be full to make any economic sense,
Again, they don't necessarily have to be full. But obviously ideally they would be.
would put a serious strain on the physical capacity there [not ever loading 900 people before on a train there],
Fair point; never having been there I've no idea what the maximum capacity is or could be.
Of course, another option would simply be to use the intermediate stations as processing points, loading a certain maximum number of people onto certain ex-London services, using whatever the maximum capacity of the station is.
would take Border staff (of both sides) away from St Pancras which is already a problem,
We don't actually know if it would necessarily mean taking staff away - it might mean being able to employ a few more, and utilise them efficiently.
and presumably at no lower fare than St Pancras to avoid cannibalising that revenue.
I think you could lower it somewhat in exchange for the added inconvenience. Fully sold-out trains (at certain times) seemingly at quite high fare prices suggests that revenue cannibalisation wouldn't necessarily be a major issue. My guess is that the fare ceiling is set by air competition rather than Eurostar's limited supply.
Presumably the large unsatisfied demand is going to be that of the lower priced tickets, which will concern Eurostar less anyway.
It would all depend - as long as lower priced tickets can still turn a profit, they ought stlll to be interested in them.
Not really. because you won't get completely full trains each departure from Ebbsfleet. So 3 departures an hour apart each will mean a security team and 2 border teams hanging about doing not very much for large portions of their shift. The majority of the traffic is already in London.
My suggestion relies on a significant no. of people who would usually go from St Pancras using Ebbsfleet instead. They get to St Pancras, get on the SE Highspeed, 17 minutes later arrive at Ebbsfleet and join the Eurostar check-in queue there instead of in the St Pancras concourse.
(Or if they happen to live somewhere where it's quicker to go a route to ebbsfleet that's not via St Pancras, of course they could also do that)