tomorrow you will get much more details on that, with a new screen. But I can reply to your questions though:
The levying is handled on the national level, thru various options costing more or less (and I'm not speaking only of money there). The frequency is variable too.
We went against removing units when the service of volunteers was finished, this would have been a nightmare to handle properly. But we are still planning some exceptional "withdrawal" of units at the start of the grand campaign to at least account the first 90-days volunteers going home. This won't be done on a regular basis though, but only in 1861 and to stress a bit more the Union player