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Well, let's try to figure it out. For anyone who doesn't want to see spoilers or how the sausage is made (so to speak), look away.

To get it out of the way, sharing your save will give me a lot of insight into what's happening, if you don't mind doing so (just keep in mind I need both the numbered save and the global.rmmzsave, or I won't be able to load the save). If you don't want to share it publicly, feel free to find me on Twitter (or X or whatever it is now) and DM it to me there. Also, I apologize if I repeat anything I've told you before, I just want to be thorough.
For the mechanical bits, it is possible to get her pregnant before the game phase changes, but you might have to try a few times to get it to happen. Deirlinn has roughly a 28% chance of becoming pregnant every time you try. If you were successful, she'll have an extra line of dialogue every time you repeat the scene while the screen is black. When she gets pregnant, the game checks the current game phase and adds 18 to it. If you have Trophy 0 from Arrival and make Punchinella active, there's an instance of her between Phoebe's house and the elder's house. She'll tell you what the current game phase is, as well as confirm whether Deirlinn is pregnant and what game phase she'll give birth in.
Once the relevant game phase comes up or has passed, the birth event will trigger as soon as you enter the final version of your Estate's yard. That means if you see weeds and your garden and pool look like crap, it will never trigger. Same with if the Freeze is active. If your pool and garden are fixed up and there's no snow on the ground, the event can trigger. So if you did get Deirlinn pregnant in game phase 0, the earliest the event can trigger is game phase 18. If you left Cumhaill after the funeral and returned, the game phase may have been 1 or 2, meaning she'd give birth on (or after) game phase 19 or 20 respectively.  For this particular example, she can't give birth on game phase 19, because the Freeze doesn't end until game phase 20.
There are a few things that could be happening that's causing you to miss the event before the second Freeze. If memory serves, it's possible to go from game phase 18 into the Freeze without going outside, and if you do that, Deirlinn's stuck waiting for the Freeze to end. The second Freeze ends on game phase 20, so if you did end up not getting her pregnant until game phase 1 or 2, that would explain why it isn't triggering. If you want to confirm that, you could try continuing on and seeing if the birth event triggers after you've completed the Freeze.
As an extra bit of information, if you're worried about Kala's bonus Freeze objective (since that requires Deirlinn being in your nursery), you don't need to worry about that locking you out of getting the second Freeze trophy. The trophy doesn't require Quinn or Teely, so if you've completed one (or both) of their extra objectives along with everyone else's, you'll still get the trophy.

I hope this helps.