No, you have to choose before check-in, you can't roll the dice and pay if you end up in a middle.

My EasyJet experience is that if I'm travelling in a two or a three (me+wife, me+child, or me +wife+child) they will allocate us in a block together. If we're travelling as a three we get the block of three, as a couple we'll get window+middle or middle+aisle. Travelling solo it's rare I get allocated a middle seat for this reason, I'll usually be allocated either the aisle or the window with a couple next to me. I usually only get the middle where a couple have tried to be clever and paid for aisle+window, so even then I don't usually end up in the middle.

But if the flight time is more than an hour I'll pay to choose my seat, I'm a six footer and the middle seat between two strangers is hell. I'll tolerate it on the jump across the puddle to Manchester or Liverpool but even the run to Gatwick is beyond my tolerance.

In terms of timing, there seems to be no rhyme or reason to it. Without paying I'm usually in the back ten rows, although I quite often seem to score a row 1 seat when I pay for a large cabin bag (no seat selection). I suspect that EasyJet operate a form of theoretical seating but, unlike Ryanair and Wizz, don't deliberately split people up.