You're correct.
I'll have to check with the experts in that field, but I seem to remember that the maximum throughput of the signaling system and track layout at Mount Dennis allows for 25 trains / hour considering 300 foot long trains.
And while it would be problematic to operate the eastern end of the line at those frequencies, recall that at that headway the service pattern would have every second eastbound train turn back at Brentcliffe, not continue on the surface section of the line.
You're absolutely right, and this is part of the problem with Metrolinx's metrics which generally rely on the manufacturer's ratings, and not observed operational limits like the TTC's.
So those numbers end up being the absolute maximum capacities, not what is projected to be the maximum capacity of the line from an operational standpoint. Just like how the TTC aims for a something like a theoretical maximum operational capacity of 32,000 passengers / direction /hour on the YUS, even though running trains at those headways (and considering a 6 passengers / metre loading) would result in an absolute maximum of something closer to 43k.
The number of doors is the biggest single factor in affecting dwell time. Level boarding, regardless of the floor height, is a close second. Once people are clear of the doors they can move around within the vehicle while it is in motion, so the internal configuration of the vehicle is a far smaller factor in the grand scheme of things.
The biggest issue with the LRVs being used in Ottawa and Toronto is that they don't have enough doors.
Dan