5E DDAL, correct? Where are you seeing these claims?
1) I am not aware of a simulacrum having any special CR given in the rules. If a rule targeting the simulacrum requires using its level, I would set that level equal to the original target of the simulacrum spell. If that target is a PC (possibly the caster themselves), use the PC's level. If the target is some other creature (with a CR rather than a level), use that CR.
1a) Yes, the simulacrum is a construct. This was done to stave off the infinite simulacrum loop, but only accomplishes that if there was limited access to the original target. If the original target remains available, you can still get infinite simulacra (though DDAL rules explicitly prohibit this).
2) In 5e there is no specific limit regarding Polymorph and constructs. The only limit is that the end result of Polymorph has to be a beast (not just any creature) up to 1/2 the target's level or CR, as appropriate.