I think your LO needs an A time increase for sure
The average A time for this age is more like 1hr45 Average A times- BOOKMARK ME!
I completely understand where you're coming from with thinking she can't stay awake longer than 80 mins or so, because I did EXACTLY the same thing at this age - and then wondered why my LO wouldn't sleep longer than 50 mins
At this age the old tired signs can become unreliable or mean something else, like needing a change of scene.
I would push out the first A time of the day and aim to have her asleep closer to the 1hr45 mark. When you get to the time when she usually starts to give her sleepy signs, have a change of scene and do something interesting but low-key to get her through the next 20 mins (walk around the garden etc). If she does a good nap here, great! Try the same A time again. If she still does a short nap, you will need to reduce the next A time so she doesn't get overtired - so maybe only 1hr15/20 (I suspect this is why you're sometimes getting a good long nap, because the shorter A time you're doing is what suits her after a short nap, if that makes sense). You might not see any difference straight away, or she may even start doing OT naps, but stick with it for 3-4 days to let her adjust.
I think your feeds will sort themselves out as the naps do - so when the naps fall into place, she'll naturally be doing a 3.5/4hr EASY. It makes sense that if she's awake, possibly crying, she'll be hungrier earlier but if she's sleeping well she should just be hungry when she wakes up. This was my experience anyway! (Although, is she bf or ff? Some bf babies aren't ready for a 4hr EASY until they are closer to 6 months). When you do get short naps, it's inevitable that your EAS order will be messed up - it'll be more like EASAEAS. Just try to predict when LO will next need to sleep, and do the feed a little beforehand so you're not actually feeding to sleep
FWIW, when I FINALLY clicked that my 14/15 week old LO needed a major A increase, our whole routine fell into place within a week. Hope it's the same for you!