Careful. You are playing a dangerous game with the seat and seatbelt. They seatbelt isn't the ideal height and is not meant to protect a child in a convertible carseat in this manner. For an infant carseat yes.
I have friends that to this day do not tighten down the straps on the carseat for their children. I have no idea why. I think the response was, "She doesn't like it too tight." But that's how the straps protect your child! I'm glad you are trying to find a solution to keep your child safe.
My son is 3 1/2 and 28 pounds, so I feel your pain. First, have you raised the height of the straps as high as you can? How tall is your daughter? My daughter was always tall for her age, so she maxed out the height of her carseat earlier this year and now has a booster, but she's 5 1/2 and well over the minimum 30 lbs for it. Last winter was tough with the coat. Manufacturers recommend the child doesn't wear a coat in the car seat. Is it possible to have her wear layers and only layer one in the car? Then put on her coat before she gets out of the car?
And a more detailed comment about the booster seats, they mostly say 30 lbs minimum, but all the safety reviews I read online say that really 40 lbs should be the minimum. Our little 3 year olds are far from ready for that. Even my daughter who runs in the 75th - 90th percentile for both height and weight only got in this year at age 5 and 48 lbs. I didn't understand why until we had to return one booster because she didn't "fit" in it. Some are really made for older, bigger kids. And we settled on the Britax (new one, can't remember name) and LOVE the extra safety features as well as the ability to take the back off when she outgrows the need for that.
Good luck and feel free to email me offline to talk more: ____@____.com.