Thanks Timo and Throwback for your replies.
1) That the Arcos was (invisibly) portrayed by the Husnock ship is not something we'd have to ignore. After all, the Husnock model subsequently became a prominent transport ship design, and although never actually seen in Starfleet or Federation hands, might have been part of the Starfleet arsenal nevertheless... I'd have no problem believing that the Deneva class in fact looks like Kivas Fajo's ship, say.
But here's the thing: The stock footage of the Arcos didn't show the ship in the Fajo configuration. It showed it as the Husnock ship. So we either have to be a literalist and state that the Deneva class looks exactly like the Husnock ship (which is patently absurd), or we just have to say that we're going to ignore what we (barely) saw on screen and consider the Deneva class to be more what we typically think of with Starfleet vessels: A saucer and nacelles. Just the Arcos by itself wouldn't be a problem, but we also have the LaSalle specifically referred to as a starship. If we believe that both ships are of the Starfleet Deneva class, then I have a hard time thinking that it looks like a wedge-shaped alien ship with no saucer and nacelles. Again, YMMV.
2) The Arcos did not necessarily have a crew of two. The dialogue could easily be interpreted as the ship having a surviving crew of two instead. There are enough cuts in the action to allow Picard to establish the number of survivors either before the bit of auditory contact we actually get to hear, or immediately after it. The shared basic design of lifepod between the Arcos and Fajo's ship is something I'd like to embrace here as well.
I'll have to review the episode again to determine what exactly was said about the number of crew, but if the Arcos was that old (NCC-6XXX), and we already had old ships like the Oberth class operating with as little crew as five people, then it's within the realm of possibility that the Arcos could be the same.
4) As for the "Antares Class Cruiser" text on the Xhosa, this actually sort of equates "Antares Class" with "Starship Class". And since the latter has to be accepted as a somewhat generalizing umbrella term encompassing the Constitution class, the former could also be an umbrella term rather than a class name. That is, the Xhosa belongs to a ship class that fits within the Antares Class parameters, much like the Nimitz class carriers today are Panamax Class vessels.
This was the one thing I was really stuck on, because of that plaque. However, because the plaque wasn't actually ever seen on screen to my knowledge, I decided to ignore it.
Or then Yates stole that dedication plaque from an Antares class vessel of that name as a prank. Perhaps Xhosa is a native Petarian name, while Yates rather preferred to interpret it as the name of the people she descended from (even thought the pronunciation is different), and stealing the plaque created the desired connection...
Why would she steal something that she could easily replicate on her own?
5) For the Istanbul to have a passenger capacity in the thousands, I'd suggest a design quite different from the known combat starship ones. But a tug with a pod would also do...
I suggested that in my essay, since I didn't like the idea of another class of ship being as large or larger than the Ent-D. Plus, if it's a colony ship, then it would make more sense to have a detachable pod with supplies to leave behind.
...and the two DS9 kitbashes with a Ptolemy-like configuration, the Curry and the Raging Queen, are thus good candidates. Not that I'd personally want the Istanbul to be one of those (what I actually think is that the Curry is of the Mediterranean class, as per the registry range!).
Yes, the Curry could be a potential Mediterranean class ship, but I'd prefer a distinct design and not an out-of-scale kitbash.
6) For the absence of screen evidence on all but the most common starship types, I'd primarily refer to the number of "transport" or "supply" assignments for the conjectural classes. Being of a design dedicated to these tasks would no doubt keep the conjectural types off the big first-wave battlefleets. Conversely, one wouldn't see conjectural combat types in TNG which takes place in peacetime; only hybrid exploration-capable vessels would be met by the E-D in deep space.
Yes, that does make sense, and would explain the frequency of conjectural classes with numbers so close to one another.
The Zodiac-class Yorktown would have to be a ship commissioned after SD 42527. Around that stardate, there was a Yorktown with the registry of NCC-20045.
Both "Frame of Mind" and "All Good Things" take place after that date, so there's no problem other than a brand-new ship after that date having a 6XXXX registry instead of a 7XXXX registry. But if registries are batch numerical as in my spreadsheet, there's no problem whatsoever.
The earliest that TMP could take place was 2272. Memory Alpha has settled, largely, on 2273. We learn in a Voyager episode that the five-year mission under Kirk ended in 2270, and in TMP that Kirk hadn't logged time in space for 2-and-1/2 years.
I just used the Encyclopedia's date for TMP, but I can easily change it.
And, there was a Miranda-class starship with the registry of NCC-4186.
Thanks, I'll add that to the list.
Finally, I don't know how you approach the Okudagram displays from "Conspiracy", but there are examples of ships with registries in the 8xxxx range, including the Ticonderoga which has a registry of NCC-87270. (I notice that you are not including ships that were seen in the Okudagrams for this episode and "The Measure of a Man".)
Basically, because even in HD the displays from "Conspiracy" and "MoaM" are still not clear enough to determine those ships' exact numbers. If I had a clearer readout of those charts I would definitely have added their info, even though they were from early TNG and some info was contradicted later.