To clarify a few of the hints for the key, it's on the third floor, and it's not hidden in an object or on a person. If there are any hints you get by talking to the NPCs that are confusing or unhelpful, feel free to point them out and I can make some dialogue updates so things are clearer. Though to save you some mental anguish, finding the key is totally optional, and can be done at any time. It doesn't really unlock any extra content (yet), just gives you access to a bit of loot and lore.
As far as getting Deirlinn pregnant goes, yes, you did soft lock yourself out, but yes you're close if she's saying she hopes to see more of you. If you didn't skip the repair quest, you can trade your leftover wood at the Cumhaill trader for more rep. If I'm remembering my math correctly, you should end up with 50-60 extra wood and (if you're getting "I hope to see more of you") trading in 30 wood will for sure unlock the scene. If you did skip it, the lock is a little harder, because there's no more content (yet) to gain more Cumhaill rep (I'm working on a solution for that so gathering extra wood still gives rep if you skip the quest, but the next releases are higher on my priority list).
If you want to get her knocked up badly enough to restart your save, make sure to do absolutely everything in Cumhaill before returning to your Estate with Phoebe. It's possible to have that scene before meeting Archibald. If you're not sure you did everything, talk to Toíche (the bald guy in the elder's house). If he starts you on the quest, just complete it (he'll talk you through the steps, too). If he doesn't start you on a quest, talk to everybody else to see if you missed any quests. The funeral is basically a "capstone quest" that lets you know you completed all the other quests available, and completing it puts you over the rep threshold for Deirlinn.