Limited edition cards are usually just that, limited edition. They can still resurface in gatcha some time later, or not. Can't really answer about squirrel, as I quit DMM PPS not long after this feature was implemented and didn't witness card rotation process for it.
It's your call on what you want to spend Pero on, just don't get overzealous. Long-term, it's not smart to buy wild cards. You will get all pink cards maxed in a few months anyway, and you will get quite a lot of R and SR wild cards from events. Keep in mind that having a buffer of Pero (500k-1kk) for SS events is a smart thing to do.
Since there's no event announced yet, there's no reason to rush in any direction or spend money mindlessly. It does not take much time to, for example, power-level one team in case of sudden Coliseum, and there's no point in having high SED outside of SS (unless you rush story, which you really shouldn't).
So my advice for now is to hoard resources. Money, Wild Cards, etc. These are your real strength. Do the dailies. I plan to only use Pero on Pink Ticket pack a day, to complete gatcha daily. Same with Silver Tickets, my hope is that event will come soon to replenish them. Daily quests and exploration allow for this and still net a considerable Pero growth.