I can't answer all your questions, but I can tackle these:
is the 9 inch Nook HD+ as good a display as the other Nooks are for text display?
The display is excellent. I have the NC and the new Glow Light. The text on the HD+ is crisper that on either of the others. It's a pleasure to read on it. Magazines such as National Geographic are just stunning.
Can the device be upgraded to the latest version after it is purchased?
From time to time, B&N provides updates which are usually delievered over the air, although you can install them yourself. I think these are updates to the B&N sofrtware, not Android itself. The techies here will know that.
This would be my only tablet and I like the idea that it has general tablet functionality along with the native built-in reading capability. From those of you with more experience, are there any down-sides to looking at it this way?
This is my only tablet, too. It has cured me of iPad Envy. I am not a power user, but it serves as a competent web browser; keeps me entertained on long car trips; works well for email. I've used it to stream a Netflix movie to our TV. With access to the Google Play store, I was ablt to download other readers like the Kindle app and Aldiko. The latter lets me access library books direactly. I bought it the day it came out, and I've been very pleassed with it since. I keep finding new ways to use it.
I believe the HD+ has the best display of any of the Nooks so no worries there. As for the non B&N content, I don't use it so someone else can address that. Now, as for the Android version, unlike a cell phone or "regular" tablet, I don't think you need concern yourself with it. B&N updates what it needs to update but unlike any other device I have, I am not really concerned with updating the version of Android. I guess that's because this is my eReader and not my primary computing method. I'm more concerned with updating that stuff which makes it work good as an eReader. Others may feel differently and it is odd now that you mention it because I'm always wanting the latest updates. It just doesn't seem to matter to me with this device. I will be quite happy if B&N just keeps improving on the eBook part of this device. If I wanted the latest and greatest in a tablet, I would have gotten something else. My goal here was to get a device dedicated to media consumption (books, movies, magazines and newspapers). At that, it is an excellent device. Like all devices, it can use some fine tuning and even some improvements to how it handles content but that's pretty much true of everything.
the display is crisp and fine to read on.
as for non-barnes &noble books, any books without copy protection/drm (I.e. those from project gutenburg) and those with adobe drm can be sideloaded on any nook. with the google play store, you can also install the apps from other ebookstores like amazon kindle, kobo, and sony.
the hd & hd+ are a heavily "skinned" version of android, but I believe it's running a version android 4 underneath. unless your willing to hack it, you'e
not going to get the "plain vanilla" version of android and are limited to the updates B&N provides. just a side note - your limited to the apps on the B&N and google play stores. you can also sideload apps through a developers kit, but I found the setup process for that tedious. there are work arounds for this, but it involves hacking the device
One piece of advice: if you're interested in magazine subscriptions, get them through Google Play Magazines rather than BN.
For many weeks now readers of a lot of magazines have been reporting that the version delivered to the Nook devices is monochrome and very dull to look at.
These magazines all have extensive interactive features that a lot of people enjoy and there's been no word from BN on when they'll resume support of those features (BN had it once, but several publishers changed formats and BN hasn't yet delivered support.)
That's true in some cases but I will say this. I went into the new Google Play Newstand and tried a copy of Entertainment Weekly I had there. I had a REALLY hard time navigating thru it as the pages gave me all kinds of a hard way to go when trying to turn them. The Nook version had no problems at all. I think I will stick with Nook magazines unless I find one of mine missing the extra media content.