I think it's her fat neck that makes her look off. Otherwise, conformationally speaking, she looks fine to me, you want the shoulder to make a O, the belly to make an O and the rump to make an O. So if you were to draw your circles on the pictures of her, you'd find the OOO that is conformationally correct.
The only reason her back looks so low is because she does have a relatively high wither, but it's her fat neck that causes it to look worse than it really is. I think she should be fine carrying weight, but as with any horse, watch for discomfort. She's got another good 10 years.