I've ordered from them with no issues. If lift gate is optional then you probably want it. If it's not presented as an option then I would assume it's included, but you might want to double-check...
I believe calibrated just means they are manufactured and tested to weigh exactly the stated lb/kg or within a tiny variance like a few grams. More commonly, plates are guaranteed to be within a...
I messed around with this recently and didn't find it particularly effective. It was fun at first but then the novelty wore off and got pretty miserable. I went back to three sets approaching failure...
Is your current split effective and are you enjoying it? I wouldn't mess up a good thing and waste time chasing "optimal." If you are wanting to make changes, I would try upper/lower for a while.