what happens if part of the last brick, perhaps including the part actually at the receive point, was already solid terrain rather than being affected by the brick - should this be accounted for, and the brick not warped accordingly?
I'm not sure what you mean here, tbh (maybe that's why I'm not quite understanding the physics issues
). Can you give an example/picture please? I'd like to try and understand this one from both sides (no pun intended)
what if the lemming is placing their first brick - should an extra brick that didn't previously exist be created below them, or should we accept this as being an edge case where the others can't follow?
From what I imagine, no extra bricks need to be created at all. The "brick being teleported" thing assumes that the brick has been placed, exists, and the Builder lem is standing upon it. The brick is then transported (or rather, cloned) along with the lem to allow others to walk out onto it.
I get that there may be important reasons why this can't or shouldn't happen, but the scenario created by a non-fix (i.e. no follow) is potentially more confusing and messy that just accepting the special case and fixing it.