It's been almost 2 months, so I'm posting an update.
Thanks in large part to your help, I've been able to diagnose and mostly fix my alignment troubles. Here is a quick summary.
The main problem, as Jim (Skylook123) correctly predicted, was innacuracy when centering the alignment stars; I've fixed that almost completely via his technique of using a (relatively) high-power EP and defocusing the star to cause it to almost fill the field.
For this, I've tried at first my highest-power EP (a Baader Hyperion 5mm, which on my scope gives 249x on a 16' field) but found it difficult to find the star, as my finder is a simple 1x red led; I've settled on using my Nagler T6 9mm (138x, 36" field) with the focuser almost all the way out, which makes it relatively easy to find a bright star after sighting it on the finder, and causes it to fill 3/4 or so of the field. Also, I keep to stars away from the horizon, like Rod (rmollise) recommended.
Another spot-on tip I'm using is Greg's (Gnowellsct): instead of trying to use the star pairs pre-selected by Astromist (which, due to the many obstructions on my observing place, is always time-consuming and sometimes even impossible), I first choose any bright star approximately in the middle of the visible area of the sky, and then bypass Astromist's list for the 2nd star and directly pick another one nearby: I've found this gives me very good precision (certainly under 10') in that area of the sky, while wasting very little time.
If I then start observing on another area of the sky, it's enough to use the "Improve pointing" Astromist menu command to add 4 more alignment stars in that region (it doesn't matter how far apart they are) and then command a "N-Star Alignment": this quickly gives me not only very good precision on that new area of the sky, but also on the previous area, and on anything in between. Just by repeating this procedure as the sky turns over my head for each new region, I keep getting great precision from the DSC the whole night.
These two techniques together allow me to have very precise positioning with almost no hassle: I've just finished a quick observation session (less than one hour, from just after the Cheese Devil was low enough on the sky to stop spoiling it, to just before the pre-dawn light made it impossible to continue), and I was able to observe a lot of small DSOs (mostly "sub-Messier" NGCs) in the Libra-Scorpio-Ophiucus area: with the Nagler T6 9mmm on the focuser almost all the way out, first I used Jim's technique on Antares and Sargas in Scorpio for Astromist's "2-Star Alignment", then I did as Greg suggested and added 4 more bright stars on Scorpio itself and did a "N-Star Alignment"; from that point on, I just refocused the eyepiece (the Nagler never left the scope) and then dialed each DSO on Astromist: after pushing the scope to the indicated position, the DSO was always on the field and, most often than not, at its very center.
I didn't need to fix my azimuth encoder's non-orthogonality, and neither to replace Astromist with better error-modeling software: just the above two techniques did it for me.
I'm planning on acquiring a illuminated reticle EP on my next trip to the US, so as to get even more precise alignment (I will post a thread about that later, on the Eyepieces forum), but this is almost perfectionism on my part, as I already have the problem mostly fixed.
Thanks again for all the great help. Jim, Greg, Rod and all the others, you rock, folks!
DSC precision troubles
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