Your posts are interesting for their analysis, but I'm one of those who thinks that Lair Assault is only one model for play. When the GM is designing or adapting encounters and scenarios for a particular group of players at his/her table, there are so many ways to respond to particular PC builds and particular player choices that I don't think there is any single form of optimal party design.
I agree that there is no such thing as an optimal party design, but I do think that just like feats and powers and items, there are better options and worse options.
As an example, I played a hybrid Ardent|Bard. Out of combat, he was very skilled. In combat, he handed out a ton of temporary hit points and healed a ton of damage when he did do a heal (along the lines of 60% of hit points straight up, more than that since he was a hybrid, he only got 2 heals per encounter and would rarely heal anyone in combat unless they were unconscious). He could also do a little buffing and hand out a free saving throw now and then.
This one PC gave out so many temporary hit points (at least 120+ per encounter at mid paragon, course, Cloak of Courage almost puts that to shame with a single power) and healed so strongly that the DM couldn't easily wipe out the party with pure damage (there were 2 other PCs that gave themselves or others temp hit points as well, so that made it even worse for the DM).
This PC arranged it so that most other PCs often had to spend only 1 healing surge per normal encounter and typically 2 healing surges per heavy encounter and he himself only had to typically use 1 healing surge per encounter. That lowered the normal average healing surge per encounter usage of the group.
So, the DM had to resort to a lot of other more unusual options (poison gas in a confined room so that all PCs had to make saves every round, heavily outnumbering PCs with non-minions, rooms with 30 foot falls into necrotic damaging muck with monsters that did 5 square pushes, dropping buildings on PCs, 100 foot falls if the players make a tactical error, etc.) in order to challenge the players.
This takes quite a bit of effort on the part of the DM to constantly come up with new and interesting ways to challenge the players shy of just bumping up an N+2 encounter to an N+3. Many monsters are more or less just designed to do damage with a small side schtick or so, and when the players minimize part of that option, it leaves a DM with fewer other options. Sure, there is always things the DM can do, but there are individual builds and team builds that can make it very difficult for the DM to challenge the players if he sticks to the normal encounter design model. Once he gets out of that normal model, then it becomes an uncharted territory of hoping that all of the effects that the monsters can do are not capable of easily overwhelming any party.
For example, a DM that pulls out monsters that can stun multiple times each per encounter can easily TPK a well designed party by sheer luck. Or, a DM that combines prone with dazed against melee PCs, or weakness with insubstantial.