it's a shame she can't manage that third sleep cycle to get to 2hrs as it does sound like she needs it - but I think you have tried a later nap to make her more tired and it didn't help with the transition so really now I think it is just a case of going with what she can do and accepting the 1hr 20 mins...you never know it might extend to 2hrs in a week or two as sometimes the change in routine makes for shorter napping than they need but gradually they relax into it. let's hope so.
As for when to get her up - if she regularly wakes at 1hr 20 and then spends a long time getting up, walking, laying down again but never falling to sleep then I'd probably just go in and get her up, as soon as she wakes or 5-10 mins after rather than leaving her getting frustrated about not being able to get back to sleep. I'll give an example of my own DS, it's a slightly different thing but it perhaps helps to know, Tracy said if they are mantra crying leave them, and that falling to sleep can take 20 mins but when I put my DS down for the night or a nap I knew that if his mantra went on for more than say 5 mins he needed help, there was no point waiting longer. The days I did wait longer he'd mantra for an hour and then scream so he missed out on an hours sleep or more. I knew if I just went back in and helped him a little he'd get to sleep on time and was still independent, IMO there was always a reason for him needing an little extra help on those days. So in terms of leaving a LO who is not doing an "I need you cry" yeah you can leave them, they are not in distress, but equally if you know a bit of help or just getting her up would reduce the amount of time/frustration then you might as well do it rather than waiting an hour or so.
I could be wrong here but maybe you can ask her - "do you want to get up?" and hold out your arms in invitation to be picked up and see what she does?
One more thing - if she continues to do 1hr 20 nap regardless of what time it is at, I would suggest just putting it at a reasonable time so that she is not too grouchy waiting for it, but also doesn't leave too long an A time before BT (basing the expectation on 1hr 20 not 2hr nap) to even her day up as much as you can. The A times do not need to be equal, some LOs like a much longer first A time and some like a much longer second A time so you judge where is best for her and set it at that time daily.
hope this helps