Ah, the carseats! We have four kids who have all been in carseats at the same time. From infant to toddler to booster size.
We have the same issues with transferring seats between my husband and my cars, grandparents, sitters, etc.
For safety purposes, the 5-point harness with a convertible system (birth - 50/80 pounds) are best - and will only cost you a one-time purchase.
I have found a technique that makes it easier with the (convertible seats):
Tilt the seat way back (in a recline position). Set carseat on the seat. Pull the seatbelt all the way out (as far as it will come out). With one hand, ensure that the belt does not ratchet back, and with the other, feed the belt through the carseat's belt holes. Snap the seatbelt into the lock. Let go of the excess belt, and let it ratchet down (into the seatbelt system). Pull to tighten - feeding as much as possible that will ratchet down. Don't worry about it being tight or secure (yet). This is where the reclining seat comes in! Now, raise the seat into the upright position. As it raises, it will begin to automatically TIGHTEN the carseat down. This saves you from pulling/tugging/re-adjusting. VIOLA!
This is great for the mini-van's back seats - you can just open the trunk - recline the seats, and feed the seatbelt through - all from behind the seat. When you get to the point of raising the seat upright, it will tightten down without you having to crawl in/out of the van.
Best,
C.