I'm thinking of starting one of my freshman over one of my seniors for the rest of the regular season, to bump up his ratings more quickly for future years. (I've got 6 freshman this year.) But, will that bump in WE still hold if I put the freshman back on the bench during my CT (start the senior to try to sneak into the NT)? In other words, is it better to give a frosh a few starts and then yank it back for a few games than to not start him at all?

Give him a few starts, ideally in a row. He won't drop any WE if you stop starting him, unless he was promised.

Yup. The improvements are after each game, so you won't lose anything you've gained or see any detrimental affect unless he was promised starts. He's just happy to be starting any games. Not sure about "starts in a row" suggestion as it doesn't matter a lick if they are consecutive or not, tho.