So I spoke to my DS yesterday. I'm on the east coast with a VIN of 42XX for a 90D, 7 seats, and virtually all options. I had a previously scheduled delivery of late April/early May. He was going to let me know when the car started production, and since I hadn't heard from him it was obvious that late April was not going to happen.
We actually had a fairly length conversation, the gist of which was I was delayed to mid-May-early June. According to him a combination of the rear seat issue, and the need to slow down production to make sure "fit and finish" was up to par caused a general slowdown in production. When I probed him about specifics, he basically said the issues reported in forums were what they were working on, and when I asked him about particular examples such as window and door alignment, seals, etc. he said those were the kinds of things. I asked about wind noise and he said that was one of the most frequent issues raised by MS owners, exacerbated by the absence of masking engine noise. I asked whether that improved over time from more refined production, he said there are fewer of those complaints with later MS deliveries. I asked about the "double-vision" issue, and he said that was not one of them because the phenomenon is not limited to Teslas, and is dependent on multiple factors unrelated to any defect in the windshield.
Bottom line, his message was the factory wanted to make sure when a car was delivered it was the best it could be, and SC visits could be minimized. Based on what I've read, if that is actually the reason it is a good thing, since most of those issues are annoying but not serious, and could largely be avoided if production were more finely tuned.
Finally, i asked him if he thought the current delivery estimate was solid. He said the shuffling of delivery dates seemed to have quieted down in the last day or two, which he took as a sign of reliability for this revised schedule. BTW, on the website it now says "late April-May." No "early" May. While I am really anxious to get the car, if a month's delay enables them to deliver a more perfect car I'm fine with that. We'll see.