I think we have to assume that the Borg only had one piece of temporal displacement equipment, aboard the Queen's sphere. Otherwise the Borg would be constantly making forays into the past to rectify past failures. In fact, they would already have conquered just about everything!
The Queen doesn't have a particular ship. She said she was aboard the cube that was destroyed in BOBW and in Voyager when she needed to travel, she used a diamond-shaped vessel. Their forays into time travel could have been a newly aquired acquisition at the time of First Contact. Borg only know what they assimilate. Perhaps they got ahold of some Krenim scientists but only slowly developed time-travel from a handful of individuals which had to filter throughout the collective.
The real answer is that ENT started off with an amazingly interesting idea, but didn't know what it was doing and ended up bringing in TNG races for no other reasons that a lazy demographics grab. Hence, why were there Borg? Because they wanted an excuse to have Borg. There was no reason for it, since the 22nd century could have been interesting all on its own (also why there was no reason to tie the series to time travel). You can create any in-universe explanation for it, and lord knows we have to, but that's the actual reason.
Really? The Borg were in ONE episode. And it fits in very well with the rest of the series. And ENT didn't bring in a bunch of TNG races. The main players throughout the show were the Vulcans, the Andorians and the Klingons, all established in TOS. And they did a huge story arc in season 3 with a race that had never appeared before at all.
In my novelization of "The Best of Both Worlds" it's the latter. Actually, I figure that the message (and the reason for the Borg going back in time in FC) was more about sending the Borg of the past information about the future.
Also seen were the Romulans, Ferengi, Orions, Tellerites. Thing is the writers showed they were capable of creating new interesting species, so why the "retreads?"
And with the exception of the Ferengi (also just one episode), none of these were species from TNG. And two of them were only ever seen in TOS until that point as well! So I don't see how the "TNG retreads" argument is particularly valid.
In addition to trying to fix a half assimilated ship, he had also sent the entire remaining crew in escape pods down to isolated sections of Earth- retrieving all those people and pods would take priority. The big question is whether the events in First Contact created an alternate timeline in the first place. The NX-01 is sometimes considered to be part of that timeline as it was unusually advanced in some aspects, taking events forward to the TOS Kelvin being so different than what we would expect from the prime universe timeline of the original series. The encounter with the Narada would have been the second contamination. When the Borg went back in time their vessel's destruction was not part of the original timeline so anything that happened to it's debris would be also part of the new timeline.
I don't think it's possible to call ENT an AU in light of season 4 episodes like "In a Mirror, Darkly" (which ties ENT directly to TOS: "The Tholian Web") and "These are the Voyages" (which ties ENT directly to TNG: "The Pegasus") There was a lot of time travel in ENT, and history definitely changed during the time war, but I think it changed to the timeline we know from TOS/TNG and the rest, not away from it.
There's at least 285,000 unique timelines that features a version of the Enterprise by the 24th-century...
It's a nice idea, but I watch Enterprise and I have no problems seeing it as less advanced than the Original Series. I don't need a multiverse theory to explain how a set designer in the 21st Century can incorporate elements undreamt of as ever becoming so compact in the 1960s. Same deal with the Kelvin. Over a century in the ST universe, the design aesthetic just went through complex and industrial, on its way to colourful, and streamlined. Simplistic to the point where one jelly bean controlled what ten calculator buttons did before. For a prequel watched before the 60's show the design and characterisation was dead on. It was the storytelling approach that's occasionally anachronistic to that setting and schizophrenic as a result. Because of having to involve the Temporal Cold War really. As much as I was sick to death of time-travel by the end, I didn't see anybody intending all that as anything other than pre-destination paradox resolved as they went along. Funny thing is, now the Abramsverse movies are commited to not doing time-travel, I kinda wish they would...
Picard saw what we saw. The sphere disintigrated into bits the size of thumbnails. He had no reason to think an intact chunk and a bunch of survivors that clearly weren't there would somehow appear.