To answer your first question, my daughter was ERF until she was 4y4m. She was right at 35lbs clothed in her cold weather clothes, but below the height limit. I went ahead and turned her so I didn't have to worry about what pieces of clothing or shoes would put her over the RF limit. At her age, I didn't feel it was necessary to purchase a different seat to keep her RF even longer. We could have turned her back RF when spring/summer came, but didn't, and she has finally passed 35 lbs now.
I plan to ERF DS until at least 4, and hopefully longer if he has not outgrown our seats rf, which I don't suspect he will as he is even smaller than DD was. He's not technically ERF yet as he's not even 20 lbs.
In your situation, if I needed to buy a new seat anyway, I would buy a seat here. There are plenty of options that can keep a smaller child rf for a long time. They are much more affordable and legal.