ATS6 continues to have me come up with backroutes while
really making me scratch my head over the intended solution (so, bad and good I guess). On the other hand, the backroutes have gotten yet tougher to execute, now requiring some pixel-precision here and there (see spoilers). In fact, technically I've confirmed neither backroutes because there's "luck" at the end of both in assigning a right-facing lemming in the crowd the fencer, and I've been unlucky so far.
Even if you choose not to eliminate them, I'll definitely continue to try to work out what you intended solution is, given how long it has eluded me so far. For once, the silence on this thread will be coming from my end, as I puzzle myself over your level.
[edit: added some additional stuff on "backroute2" that you might want to watch out for, see spoilers]
backroute1: your terrain changes almost eliminated this "backroute which wouldn't die": the solution where a spear is thrown for later glue-pouring, and you start pouring glue while the basher/fencer is almost but not completely broken through to burn off some glue-pixels. Nevertheless, I still found a position for the stacker that works for ATS6, but now some moves need to be done at pixel-precise locations. See screenshots B and C in "ATS6_backroute1.zip". I wasn't able to fully confirmed this solution because of the luck required in selecting a right-facing lemming for the fencing, so I'm not 100% sure whether the glue burn-off still works out correctly now that the basher is replaced with the upward-sloping fencer.
backroute2: new backroute not possible in previous versions of level. Yet probably not hard to eliminate? It's very similar to backroute1, except the stacker starts further to the right and serves to trap everyone (screenshots A). For glue-pouring on the spear, we now use the last hopper to send one more lemming up to the top, and have him pour glue at the precise position shown in screenshot D. This will burn off just enough glue pixels so that the remaining glue that falls down onto the spear and extends off of it will just barely fall short of reaching the green triangle (screenshot F). As seen in screenshot E, the glue-pouring lemming will then stomp down so he can fall safely onto the spear (as well as removing the wall blocking the crowd from reaching the spear). Free the crowd by fencing through the stack.
[edit: additional variations on backroute2 you may want to look out for. I haven't even tried any of them yet but they look plausible enough that you might want to take them into account when doing backroute elimination. (So bad news, maybe not that easy to eliminate after all...)
a) by starting to pour a little earlier, you can burn off even more glue, but the glue will block the lemming and make him turn left. He will then walk up to the dog (I think) and fall off of its nose. Looking at it in the editor, I think the fall is not fatal. If the stack started a little bit further to the left, he can then turn around at the stack, head right, and stomp away the wall without starting all the way from the top of the glue.
b) even if it's not possible to move the stack further left (so that when the lemming falls off the dog's nose, he merges with the crowd), an alternative is to have the last lemming out start fencing at the bottom of the valley left of and closest to the crowd's pit. My hope is that the fencing can continue until it breaks through both the left wall of the pit as well as the stack. That way, the crowd will head left initially (I guess depending on how the fencer breaks through), isolating the fencer to do the stomping-away of the wall.