Yes you do need to save yourself from the frustration of trying to sleep train or extend a nap with an UT LO, it's agony!
Your first post you said you'd increased to 1hr 40 for the first A. I would go with that for now, actually if you have the courage you could increase again, I say that because I actually prefer a bit of OT than a bit of UT.
Obviously you have your other children to care for. If you think you can manage a W2S at 35 mins through to say 50 (this is shorter than the usual length of time suggested) then this could be a start, just do it for the first nap not all day. W2S can take time, but it can also be amazingly fast. I managed to time my DS's W2S so well that I went in at 19min to see him through an OT 20 min WU, I was out of his room by 21 min...went in again at 39 min and out by 41 min, guests in the house didn't even know I was doing it! if I didn't go in he woke bang on 20 min screaming blue murder and took 30 mins to calm down, totally wrecking his nap. He was much older so it is a different circumstance but I'm just saying it as it *can* be possible to nip in and out if you're lucky and if you get the times right.
Yesterday she did her first two naps at 45mins and then slept 1hr50 with an A time of 1hr25. That was the same A time from her 1st nap but she only slept 45mins that time.
My guess is she was not tired enough having 1hr 25 following a long night sleep. By the second nap she had accumulative tiredness from two A times plus a short nap so she was more tired, more ready for a longer nap. If you extend the first A and the nap extends you may need to increase the second A the same day to try to keep her tired enough for that second nap,. don't assume the same A time will give the same result as she will be more rested.
I know it is confusing. Sorry.