Some of your suggestions ring true for us, some do not.
In the past 2 hours total nap time seems to be optimal for him. This was composed by 1.5hrs one nap and 30 minutes the other, with him him alternating these (more or less at random). I have have had limited success with resettling and cycle blending post 6 months. I find that, if more sleep is needed, leaving him to make the transition between cycles or shhh-ing through the monitor is enough to get him to get him back to sleep. Failing this, nothing works (including the opportunity to dose off at the post-nap feed) and I have concluded that if he doesn't resettle easily, he simply isn't tired enough. I should also add that, most of the time, he wakes very happy and easily gets to his next sleep time without obvious signs of OT.
All of this having been said, on the days that he gets 2x catnaps, he also copes quite well. A few days of it, lead to the OT signs, 1 day like this is manageable. I do however makes sure he gets to bed early on these nights.
I don't think he is OT due to a sudden push in A times. He was not OT when the A time was 3.5 hours and there were days when it pushed out to 3.75 or 4, even 4.25, on occasion due to life circumstances or him taking a little longer to settle. So.. the push to 4 or 4.25 isn't a sudden or big change in A time for him. I usually judge the OT/UT situation by both his ability to settle easily and the length of his nap. The 30 minute sleep seems to be most characteristic, for us, of UT with OT sleeps being more like 18 minutes and often requiring settling support.
I do however agree that slowing the increments would be advantageous to give it a few days for any changes to shake through. I did however have a little panic about the possibility of going to one sleep (and having a huge strength in the afternoon if I didn't push the morning nap back) and/or having to get to 5hrs A time (as you originally suggested might be needed) from 3.5 before his naps would improve.
Our sweet spot after the CN (before bedtime) seems to be 3.5. This also seems to be the case if the morning nap was a CN. If I pull the bedtime A any earlier, I get major settling difficulties. I see this for an A time less than 3 hours unless the nap situation was extremely low for that day. Anything over 5.5 hours results in feeding to sleep at the bedtime feed and around 5 hours is often an OT settle, where I need to shhh-pat or similar.
I hope that isn't information overload. I just wanted to highlight the characteristics of this individual and how they are impacting on the struggle to identify his ideal A time at this point in time.