To the OP... DME and DCL buses do not have seat belts so you are unable to secure a car seat in either. Since you don't forsee using a smaller vehicle w/ seat belts, using the CARES harness on the plane and skipping the car seat is probably your best bet.
About the CARES harness...
It is a great option if you will be flying often enough to use it. It is pricey ($65.95 on
Amazon) so if you will only be flying once or twice while your child is between 22 and 44lbs (and 40" tall), it may not be the most affordable option for you.
For us, ds only flew at 2yo (31lbs) and 4.5yo (46 lbs) so it really didn't make financial sense to purchase for just one trip. Just as easy to take his car seat w/ us to use on the plane and then put it under the DME bus and in the corner or our resort room during our trip.
If you travel often and don't need to use a car seat at your destination...the CARES harnesse is a great alternative to bringing a car seat!