Can't speak to the pickup question,seems like the poles might be a factor, but it looks like they are actually staggered pretty evenly, they might pickup up the strings well enough.
I wonder what the width of the neck pocket is.
It might be possible to use a four string neck, center it and just shim the sides of the pocket.
That way the nut and tuner holes would be aligned. Just swap out the bridge for a four string.
I'm doing something similar with a mis-matched heel size - putting a Squier mini p neck in a larger pocket of a Solo mini bass b stock body.
I just put some layers of plywood adhesive banding (iron on or use heat gun) along the sides of the body to make it snug. Easy to trim down with an exacto knife once you put it in place. You do end up with gap of the pickguard aaround the neck, though.
I go the budget route, and that's sometimes the downside.
But at 30% off now, that puts it below $100 for the whole loaded body.
Amazing price, unlikely to see anything like that again.
Or...you could go with the Ursa 5 string neck, (last one at Rondo) and just go with that.
Love This : SX Bass Neck for Ursa 2 MN 5 String
www.rondomusic.com
You could de-fret it if you wanted fretless, or go with one from ebay, as you said.
