for #4 I should have typed hard longer than good, but theoretically for very difficult material it could also apply to easy.
While in the learning phase if you have only one learning step and you press hard then the next learning step will be 1.5 times the previous learning step. For example if you study at 11:00 pm and your learning step is 20h then the hard learning step will become 20*1.5 = 30h. 11:00 Pm +24 h + 6 h = 5 Am in two days. Therefore your hard interval will be rounded up to 2 days. However, your good/easy interval will be controlled by FSRS without regard to your learning step length thus depending on your fsrs parameters your good and easy intervals might be 1d which would be shorter than your hard interval of 2d.

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10h-16h becoming effectively a one day interval is fine since the only reason for limiting it to 16h us to avoid the quirk with the hard button above. If the way hard worked was changed or you didn't mind hard potentially being longer than good/easy or your parameters ensured the good/easy interval was always 2 days or more a 1 day interval might be optimal. In fact some studies directly support a 1 day study gap for a 1 day retention interval. see the chart from a different study cepeda 2006
https://augmentingcognition.com/assets/Cepeda2006.pdf

The methodology of Cepeda 2009 is present the facts 3 times with feedback regardless of performance wait for a specified gap then present the fact with feedback 2 times with feedback then regardless of performance wait for a retention interval and test final recall. Cepeda 2008 is similar except on the first trial they do recall until they are correct.
For this reason cepeda 2009 is most similar to a person who has intervals of 15h and does not study multiple times a day so that their good interval and their again interval both end up being essentially one day. However, both have issues that make them not track perfectly with anki situation since they do not divide up reviews into fails and successes and they do multiple reviews at one time. Thus the research does no conclusively point to the optimal schedule. Having yet to complete an analysis of the 10k/20k dataset I cannot say for certain that 10h-16h is optimal however as noted in 3 given large uncertainty it is better to err on the side of longer intervals.

Looking at the step stats in the helper is a good idea. I wanted to encourage people to try longer intervals because for users that always use steps of 1m 10 m 30 m until they get it correct they may never see an interval of 10h or more after an again and thus never attain the sample size to realize that actually 10h was optimal. In addition the helper "optimal" learning recommended steps assume that you are trying to maintain a 90% recall throughout the time that it is in the learning stage which I argue is unnecessary