define multicycle path on specific instant

for a start, are you sure the data will be aligned correctly? you need to make sure the data is set up 2 clock cycles before the enable is high to ensure that the setup time wont be violated with these slack paths.

It might be easiest to just tell it to look for all registers in the path with wildcards:

set_multicycle_path -from [get_keepers {ent_name:inst_name|my_slow_entity:slow_entity_inst|*}] -to [get_registers {ent_name:inst_name|my_slow_entity:slow_entity_inst|*}] -setup 3

set_multicycle_path -from [get_keepers {ent_name:inst_name|my_slow_entity:slow_entity_inst|*}] -to [get_registers {ent_name:inst_name|my_slow_entity:slow_entity_inst|*}] -hold 2

This will then only get applied to valid paths - any connections that dont exist will be ignored (so if you have the registers A -> B -> C, it wont set up a path from A -> C because that connection doesnt exist).