Okay, I see that what I'm getting at is being misunderstood, so let me try again.
Where I spoke about L3D, I'm specifically talking about visuals
of the lemming. Now - Proxima did propose in one post that this should be outright instant, which would imply no animation (but still maybe a sound), the lemming instantly changes location upon entering the portal. In that case, there
is no "lemming visual" to speak of, and this discussion point would be redundant.
I was envisioning it more as like in Lemmings 3D, where the teleporting lemming temporarily enters a "teleporting" state. (We might want to use a different term instead of "teleporting" though, for obvious reasons - "warping" is one option.) This state has its own graphic - this is independent of the graphic of the teleporter itself, this is the graphic of
the lemming. Here's a video - ignore the teleporter itself and just pay attention to the lemmings:
https://youtu.be/KrgP7BcBUc0So my thought is - when the lemming encounters a portal, an animation similar to this occurs while the lemming teleports, which takes a few frames. The portal would
not be sensitive to releaserate or grouping; the
entire point of the portal is that it differentiates itself from the teleporter in this way (essentially, portal is to a teleporter, what fire is to a trap); on a physics level it's like a trigger that puts every lemming that enters it into this "warping" state immediately (much like how fire puts all lemmings that enter it into the "burning" state).
As for whether the lemming comes out the other end continuing the current skill or as a walker/faller, I'd prefer "keeps the current skill" unless there is a good reason not to, simply to keep it consistent with teleporters.
* Two-way portalling is problematic because we obviously don't want a portalled lemming to warp back on the next frame. If it's possible to get around this problem, then I think it's a good idea -- if the portal is essentially a "wormhole" connecting parts of the level that are not next to each other in normal space, then it would make sense for it to be two-way.
Splitters have this problem too if a lemming drops in from above; it's not at all difficult to deal with. It could get messy if a level had multiple overlapping portals, but that to me feels like a "do not do this".