We love love love WL - and the ramp from the lobby down to Roaring Forks QS is an E-ticket all by itself!
You are correct; the boats at WL alternate between the launches and the bigger boats that can accommodate mobility devices.
We never had to wait longer than 20 minutes for another boat to arrive, BUT having said that, I have seen a few folks around the DIS complaining that boat service was not consistent earlier this year during the late spring/early summer.
We found that the bus ride from WL over to MK and back was usually pretty quick, and averaged 30 minutes or less. It does seem to depend on the time of day - we have ridden back to the Resort in the middle of the day, and found ourselves on what we jokingly call "The Deluxe Tour", which stops at either Poly/GF or MK first to pick up more folks - however that was during slower times of the year (January).
I highly recommend WL - if you love AK, you will love WL too; they were both designed by the same architect!
