If I request books before Feb 15, and they roll over to the next sender (say someone doesn't respond, etc) before that date, will I be charged the 49 cents?  If the transaction isn't complete (marked received) before the 15th, do I get charged the fee retroactively since the transaction isn't done?  I just need to know how much PBS money I need to keep, just in case.  

while I can't say definitively what will happen here, when the fee system was implemented recently at SwapADVD any request made prior to the change did not have a fee assessed, regardless of when it was accepted or how many times it rolled over to the next sender before finally being accepted.

A transaction doesn't have to be complete (marked received) before Feb 15th for it to not be charged the fee, the fee is charged at the time of the request. I am not sure about roll overs and if that is going to create a "new" request triggering the fee. I will see if I can find out.

ETA: and keep in mind, the 0.49 fee only comes into play if a member does not sign up for an annual membership (or after the first 30 on the Limited). If you sign up for the membership or earn Free Swap Fees with printed postage, you won't need to worry about that fee.

Letting requests roll over on purpose is just being mean imo

Yeah, just being mean is what some people are doing just to be spiteful right now unfortunately. Hopefully they will get bored with it long enough before the 15th to not interfere with orders right around that date.