For us personally, DD will be staying in a 5 point harness as long as possible. It can be tricky to get a proper fit with an adult seatbelt on a booster seat, not to mention getting the child to sit up and wear it properly.
My Dd is 17 months right now and still rearfacing in her Britax Boulevard. This convertible seat has a 5 point harness and a weight limit of 65 lbs forward facing (33 lbs rear facing). It is true that once she reaches 40 lbs I am supposed to switch from using the latch system to using the seatbelt to secure the seat. I think that might be what a previous poster was alluding to. I've never heard of a 5 point harness having a weight limit, other than the limits of the carseat.
When Dd outgrows the Boulevard, we plan to move her into a higher weight limit, 5 point harnessed booster seat. No matter how tiny the risk, for me, it comes down to asking myself if I would have no regrets in a worst case scenario.
As far as the winter coats go, the best thing you can do is either remove the coat in the car or unzip the the coat and pull the sides open and away from the child's body so the harness straps come down over the child's shoulders and the coat is out of the way. The padding from the coat makes it hard to get a proper tightness in the shoulder straps. This extra give in the straps can lead to enough slack to allow the child to come out of the harness. To test, put the child into the seat with coat on and zipped. Make the straps as tight as you normally would. Remove the child and take off the coat without loosening the straps. Return the child to the seat and refasten the harness straps without adjusting the straps. If the straps are looser than you deem acceptable, you'll need to figure out an alternate plan.