The big advantage of the top down furler is the spi deployment at the top mark. The spi can already be hoisted and start unfurling it before you are round the mark such that by the time the rounding is complete you are off and running. This can amount to big gains against someone who has to wait until they have rounded to hoist.
At the bottom mark you need someone to load up the torque rope by pulling on the furling line well before the mark.
When you want to furl, let the sheet off and pull like crazy on the furling line, the sail starts furling at the top first and works its way down. When it gets to the bottom it needs some light pressure on the sheet to get a good tight furl.
I think 3 would be the ideal number of crew however it could be done with two (ie. skipper + 1) if the skipper controlled the sheet while the crew pulled like crazy on the furling line.
It probably gets more down to how energetic the crew is rather than how knowledgeable they are provided the person (skipper) controlling the sheet knows what they need to do.
You can leave the sail up and furled, but it is slower. We bring the furled sail down once we have rounded the bottom mark as the wind brings the sail back to the crew on the nets that can direct its drop as someone else controls the drop with the halyard. (If you tacked and then dropped you probably wouldn't need the crew on the net to catch it as the breeze would blow it more toward the mast and may come down on the cabin.) I have made my pulpit split at the front and the crew just flicks the furled sail inside the pulpit and then ties it down to the tramp so it does not go overboard on the upwind leg. This furler system is safer and ensures the spi does not become a sea anchor because of a bad drop. The attached pic shows the screecher in the dropped position. You can't see it but on the port side the spi is the same. Flicking it into the pulpit won't be such an issue when I get the forward nets on. I'll be fitting velcro straps each side on the main nets to hold down the head of the sails. Quick attach and release and keeps the sail on the nets and the swinging spi halyard away from the cabin top winches.
I hope this run down helps.