Or should it be expected to immediately declare a mayday once you elect to divert at final reserve + alternate?
Why on would you?
Say you divert on CMR (Altn+Fin Res)
At this point you should have already been well comfortable with the possibility of a diversion close to the limit. Maybe you should have diverted earlier if you think the situation warranted it. Maybe it's just not your day. Anyway...
You divert.
If ATC cooperates and everything goes to plan there is no need to declare ANYTHING.
If ATC does not cooperate, you declare a PAN, because you think you may end up below your 30 minutes final reserve.
If ATC still does not cooperate, and you know you will by now land below final reserve, yes, then it is not your day and you declare a mayday.
That is how the system works and if you play it like this, who can blame you? (assuming you did all your preflight planning correctly?)
Anyway, is this not exactly what the extra 30 minutes are for? i.e. to cover the fact that it just might not be your day?