In-flight Seat Change
 

Am booked LAX-MCO 19May originally through AS operated by VX in Main Cabin.

At the seat selection option, only middle seats were left. However, when I look on the VX website, both window and aisle seats next to me are available for an additional fee.

Can I skip the fee and just slide over once the plane door closes? Or will the VX flight attendant charge me in-flight? Or, should I just ask at the gate and will they charge me then? :D

Alaskaair.com will only show you "free" seats.

I fly between MCO and LAX a lot - these flights fill up. You can also get the 1st exit row for cheap compared to MCS. If you have status on Alaska, give them a call to see if you can get a better seat. Its a long wait on hold, but its worth it.

In the same row? If there are empty seats no-one will bat an eye if you shift one to the left or one to the right. If you move rows, they'll probably ask you to move back.

Those paid seats become free 24 hours from departure.

Excluding MCS.

Yeah, and excluding first... MCS is a different class - OP was clearly asking about the paid seating selection in regular main cabin.