Those already marked resolved are unchanged.
"Quest for Kieran": Flip left, for the reasons namida gave above.
"PoP YoR ToP": Flip left.
"The Long Way Around": Flip left. As well as consistency with other levels where the exit is to the right but steel enforces going left first (The Far Side, Be more than just a number), there is the consideration that being able to bomb into the terrain at the start is a consequence of NL having untimed bombers, and wasn't intended to be possible.
"The Far Side": As above, flip for consistency.
"Emmings! (No L"): I will leave this as-is (top trapdoor flipped, bottom trapdoor not). If the top trapdoor were the only one, we would certainly flip it, so the presence of another trapdoor shouldn't change that decision. The lower group typically build to the left from a point to the right of the trapdoor, so leaving this trapdoor unflipped lets them reach that point sooner.
"The gate trap lemmings": Flip left. The solution of saving the right crowd by sending them over the top is still possible and not very different.
"The race against cliches": Flip left. Again namida makes a good point about this level.
"SEGA Four": Flip left. There is still plenty of room to get down the first drop by mining from directly under the trapdoor.
"Creature Discomforts": Flip left. With two trapdoors on opposite sides and an exit in the middle, it feels especially right to have both trapdoors face the exit; and as namida points out, this doesn't really affect the climb-bomb solution as you would need a blocker anyway.
"Hello John! Got a New Lemming?": It does make sense that the trapdoors face opposite ways here, so I will flip the top trapdoor.
In short, my decision for all the "still under consideration" levels is to flip them