Yes, the Seattle Spinco - Micro Focus Intl merger step made things a bit weird.
As far as I have figured out, the issue with the "capital gains" and the missing 1099-B for that portion, is that the merger was between a US company and a foreign (UK) company. That made the merger *taxable* , so you need to report capital gains on the merger (with a caveat ... next paragraph). It wasn't a "sale" per se ... but a taxable merger. (When a US company merges into a US company, generally the merger is NOT taxable.)
The caveat is that you have to report something on the Form 8949/Schedule D about the merger only if the value of the new Micro Focus ADR shares is larger than your Seattle Spinco cost basis. (So you actually have a capital gain.) (This sounds like your situation.) (Either the HPE or the Micro Focus "tax information" document (I think the HPE one) stated that the value of the Micro Focus to use was $29.34/shr).
If someone had a *loss* then they do NOT report a capital loss ... they are supposed to just reduce their basis by the amount of the loss, but report nothing on their taxes this year.
There was a website having to do with "HP alumni" discussing all this with reference to Micro Focus derived from ESPP shares, and apparently that is a bigger mystery yet.
This, I think, put the brokerages in a bit of a bind for clients who had non-covered shares. Even if you had reported a cost basis for the HPE (or predecessors) to them, they couldn't be 100% sure it was correct, so they couldn't be 100% sure whether they were really supposed to report something on the 1099-B. (If you turned out to have a loss (not supposed to report), but the brokerage had actually put the transaction on the 1099-B, then it would be a mess with a transaction reported to the IRS that should have never been reported.
Vanguard brokerage's decision (as far as I can surmise ... I didn't ask them) was that they didn't put anything on the 1099-B for non-covered Seattle Spinco shares and left it up to the owner to figure out whether there was a reportable transaction or not. (The transaction does appear on the 2017 "realized gains and losses" tab of the "Cost basis summary" page on the Vanguard website, however.)
I reported it on a Form 8949 with the "F" box checked. ("Long-term transactions not reported to you on Form 1099-B.") Don't remember exactly how I did it using Turbotax, but it was something that was obvious to me. (Answering "Do you have any additional stock sales not reported on a 1099-B?" or some such.)
And, then, yes, there was the cash-in-lieu for any partial share of Micro Focus that arose out of all this. (I did have an entry on the 1099-B for that, because that was a true sale where either gain or loss would be reported.)
I have a note written on my spreadsheet to the effect that neither the HPE nor the Micro Focus tax information letters ever explicitly said whether the old acquisition date was supposed to be carried forward. However, the HPE letter said a lot about making sure you went through this "Do I have a gain or a loss?" decision with each lot individually, so I surmise that, yes, the old acquisition date carries forward. (There wouldn't be separate lots any more if the old acquisition dates didn't carry forward.) (Kind of makes sense since it wasn't a "sale/repurchase" per se, but a taxable merger due to one side being a non-US company.) Vanguard brokerage thinks so, too, although I take their opinions on such things with a grain of salt.
Ah, well, that was probably a lot of words for something you have already figured out. But at least you know you have company in this particular aspect of your tax preparation