First off I am happy to hear you want to keep your child harnessed. A harness seat is safer at her age then a seat belt.
My daughter has been in her regant since 3yr 6mo. She is now 4yr 6mo 44in tall 37lbs. We are still using the Long seat belt install. When my daughter hits 40lbs we will be using short belt path. She loves her seat. they is no base on it so it will sit low compared to your current seat. I have it in a Honda Pilot 06 driver out board using long seat belt install. I have also had passenger out board seatbelt. Third row out board driver side seatbelt. All were rock solid installs. I have not tried it in the middle because I have another child. I use the seat belt becuase I get a better install then with LATCH. It is a beast to move but I love knowing my child is as safe as possible. From what I have learned on car-seat.org, britax will no longer be making the Regant because the Frontier is replacing it. So you may be able to get a great deal on it. IF you are able to, go to babies R us and check it out and try it in your car. I have not had any problems with the harness twisting on me. Depending on what car you have, you may use LATCH up to 48lb. I only know of Honda LATCH limits are 40lbs because that is the car I drive. Also you must use the TEATHER at all times no matter what the childs weight. I do not own a frontier, however i did hear about the problem it had. The problem was the harness was coming off the splinter plate. They have replaced the plate on the newer models. I have also heard on car-seat.org it can be a pain to install because it may move more then the 1in allowed. I have had to call customer support 2 times on my other britax seat I own, and both times i have not been on hold more then 1min. They are very nice and helpful. I also own a Graco Nautilus. I am not in love with it. I have a hard time installing it in my 2 vehicles, honda pilot and volkswagen passat. Both cars have leather seats. I have gotton an accetable install with less then 1in movement, but it is not a solid install like the regant and my britax wizard. I get a great install with the Nautilus in a dodge caravan with cloth seats using LATCH. It does not budge. I highly recomend car-seat.org for you and other parents here. There are Certified Car Seat techs along with people like you and me. They can help you decided what seats would work best for your child and your car. some car seats do not work well in all cars. the safest seat is one that is installed correctly and used right every single time. Sure Nancy Grace has Radian car seats for her twins, but she does not use them right. the harness is above there head and they are FF at less then a year. They should RF untill the maxium of the seat which is 35lbs. good luck to you and your daughter on finding her new seat!