I was going to suggest that that might be playing a part - I'm sure that we have had that here. Not entirely sure if we are over it, but things have improved since you started this thread - I don't think we are ever going to get good naps, but we seem to be back to a point where I can normally count on an hour or so and sometimes it is more. Nights are not dreadful - he is normally getting to 5ish without needing any intervention, so I can at least have a good chunk of sleep. So hopefully miss m is heading that way too.
One thing that I do wonder - can you put her to bed super early (regardless of how long her nap is) and what happens if you do? I know with the girls that sometimes the ot was so deeply ingrained that I needed to toss out the rule book - with them it was letting them nap for as long as they wanted (as I always had to wake them from naps) and put them to bed at the usual time so their a time after their nap was only 3-3.5h, but doing that once or twice seemed to catch them up and they would go back to normal. I know that m isn't really a long napper either, so I am wondering if drastically cutting her afternoon a time by putting her to bed really early might achieve the same thing?
The other thing that I started doing with the girls at about this age was counting the a time for the day rather than morning and afternoon - so I figured that they needed 10 hours a and 2 hours nap in a 12h day, so if they were up for 6 hours in the morning then they could only cope with 4 in the afternoon. Maybe worth thinking about if you are having trouble fixing a times or nap times?