Glad it's resolved but to answer the general question anyway...

First, check again first at OLCI and then at the check in desk if two seats together have become available. No, check in staff won't move someone else for you neither should they, but 1) some people may cancel or change their plans at any time ,and 2) CE is not fixed size so some rows may be added any time.

Second, if nothing is possible ahead, take a seat that would enable you to sit together with a like for like switch, and then as others pointed out, take your seats normally and then ask the person in the less good of the two seats whether they would accept to swap. In other words, a) do get a window and an aisle so that you can offer aisle for aisle or window for window and b) if your seats are - e.g 2A and 6C, you ask Mr/Ms 6A whether they would be ok moving forward to 2A, not Mr/Ms 2C whether they would be ok moving backward to 6C. Same if one of the two seats you have is in row 1 or at the emergency exit, you ask the person in the other row whether they would be ok swapping.

As also noted, explain pro-actively you completely accept if that is not convenient and do not insist if that is what they say.

But basically, as you asked, that I think is not only the most polite but also the fairest way to ask, ie it wouldn't be fair to ask before checking as much as you can if you can sit together without anyone else needing to move, and it wouldn't be fair to ask someone to take an even marginally less good seat so you can sit together, instead, they should get a marginally better seat in exchange for helping you.