And so, after a full day of work at -1 degrees, I could confirm it was the pump after all 🤬
Here is a video taken yesterday of a cold start with the "refurbished" pump:
https://www.youtube.com/watch?v=_l-emfG-GzQ
And here one with the old pump, taken after I swapped it back in:
https://www.youtube.com/watch?v=Q6QqJHvTogs
So much time wasted because of this.. at least I know what the problem is. It's a really bittersweet feeling though, since I spent so much time and money on this engine and I now know it should have started without hesitation on the first attempt almost a month ago. All this fiddling and letting it run as it did cannot be good, but it is what it is.
I'll now see what the seller has to offer but I'm leaning towards getting my money back and heading somewhere else 😕
Thanks again everyone for the invaluable tips, I likely wouldn't have managed without you guys 🙏