Wow, great work caffeine. I have some things to add.
Free-form has better stats than stacking for tetrises, but you neglected to mention line-clear delay, which exists in many Tetris games. Line clear delay is one of the biggest reasons to go for all Tetrises with no skimming.
I've talked about this before on chats and stuff, but when your goal is to make a 40-lines
record, it doesn't really matter how many times you fail. For this reason, even if you can't pull it off once in a hundred times, you can just think of all of those as practice. Keep shunning hold and stacking the best way you can unless it presents a problem for you processing-wise.
And processing is the other reason that what you termed free-form stacking is maybe not as good as its stats claim. I can't quite put my finger on it, but I've always found it to be harder / slower to play that way. I suspect it has something to do with visual perception; dropping pieces and having some of them disappear somehow makes it harder to re-orient, even though the stack shape is no different than if the lines didn't go away. On top of that, there's vertical distance to look between your next pieces. I won't count soft drop speed here since I am talking about the optimal 40-lines game. It may very well be that this, like 6-3 stacking, is just something that can be "gotten over" with practice and work, but I don't know that it would be worth it even if you could do it fluidly... and there's something rather more satisfying about spamming Tetrises anyway

Anyway, it would be interesting to see if someone could get good enough at the non-standard "free-form" style play to make significant 40-lines progress, but even your own measurements seem to suggest that 6-3 with just tetrises beats it.
With regards to multiplayer, consistency becomes more important than speed, but on the other hand you aren't stacking 40 lines off the bat. A better argument against using this in multiplayer is that Tetris starts are the least efficient and you'd probably benefit more from learning other openings.