I've flown G4 a few dozen times, never paid for a seat assignment before. I've had a mixed bag of experiences that make it difficult to know exactly how their seating algorithm works.

To answer your first question, if you check in at T-24 you will be free to select any seat that hasn't already been selected, but it won't be free. IME I've never seen seat prices go down during the OLCI period. What will happen is that when you check in it will ask if you want to pay to select a seat. If you decline it will then assign everyone in your party seats. About 25% of the time I've had them assign 2 seats together. I've never had it assign 3+ seats together. It will then offer the chance to pay to change your newly assigned seats. It seems that they intentionally assign you seats apart hoping that they will then get you to pay for at least one seat to sit together.

One thing I have noticed is that seat assignments start in the back and move forward. You can see an example of this if you do a dummy booking for a flight less than 24 hours out and you can watch the seat map move forward as the rear seats get taken.

You will actually have better luck with the individual who is one a separate PNR. If you have that individual check in immediately after the previous group checks in, you have an excellent chance of at least having that one person be next to them.

That's probably way more than you want to know, my apologies. This is something I've spend more time than I'd care to admit trying to figure out.

My recommendation: Wait until around T-4 or later to check in. If you care to, check the seat map periodically through doing a dummy booking to see how far up the seat assignments have moved. Wait until the next available seats are an empty row and then check in everyone in the group starting with the group of 3 and then the other. You will likely then have 2 seats together and 2 apart. You can then decide whether its worth it to pay to move the other two.