multi case_sentence cause delay

250 MHz is fast enough that one or two LEs in a logic chain play a role. This answers my first question. The difference between 250 and 260 MHz isn't more than 0.15 ns delay, rather a result of different routing than a full LE delay step.

My primary assumption is, that equivalent logic synthesizes to the same gate level netlist. Thus I also assume, that your two variants aren't logically equivalent, but it's most likely not a matter of nested case constructs.

If the constructs are equivalent can't be seen from one or two lines of code.