Guess you've noticed that the trike scrubs down the crown of a bike tire pretty fast, that's the reason for mounting rear tires on the front, since they have a slightly thicker tread depth. Reason for the reverse rotation has to do with tire construction and difference in road force applied during acceleration forces (rear) and braking forces (front).
Either way doesn't seem to affect safety, although rear tire reverse rotation seems to be the way most trikers mount them. Bike mechanics don't generally ride trikes, so reverse rotation is a foreign concept to them. I haven't noticed a difference in performance on wet roads, but then again the rake angle and front end weight can have an impact on this.
As for radials, the heavier the bike, the better off you are with bias ply on the front, due to sidewall construction. Radials might work on the lighter Sportster (I really don't know).
As usual, just my 2 cents, but there seems to be constant controversy on this topic. Just be sure the tire design and profile is close to the manufacturer's spec for you bike.