Great to hear!
1 - yes, most plastic cutting boards are HDPE — inexpensive ones may be LDPE
2 - I wrote up a bit at https://www.shapeoko.com/wiki/index.php/SO3_Function_Test#Distance_of_travel and Notes on rapid positions and wasteboard leveling and c.f., Wasteboard Plans with threads
The tramming in a lower section is actually something of a problem — that’s one of the things I like about my previous wasteboard / T-track setup w/ the loose pieces — I’d cut and re-arrange them and cut them so that the sections outside the cutting area were lower than the cutting area, then I could shim up to make clamping work as necessary, but if I had a slightly oversize piece it would align to the level of the trammed working area.