After chatting with some other Knight Rider friends, I have decided that Devon is Garthe's biological father.
We all know that Devon likely had something going on with both Wilton and Elizabeth, it would not surprise me if they were all screwing around with each other at some point. During this period of time, Elizabeth finds out she's pregnant and, in true Mamma Mia fashion, she doesn't know which one of them is the father. This being the late 1940s, it's expected that she'll marry whoever the father is, and since she doesn't really know, she ends up choosing Wilton (but they all know there's a chance Devon is the actual father).
I feel like this all just adds an extra layer to so many different relationships in the show. This becomes part of the reason that Wilton doesn't love Garthe as much because there is a possibility that he's not his. And for Elizabeth, Garthe is a reminder of the love that Devon once had for her. And that would add an extra layer to Wilton making Michael look like Garthe, not only is he now the kind of son Wilton wished Garthe was, but now he finally has a son that is definitively his.
This would also make Devon's line to Wilton about how Michael "looks a lot like a younger version of you" pretty sad. All these years later and Devon is still trying to convince his friend (and maybe himself) that Garthe is Wilton's.
As for why I say that Devon is definitively Garthe's father, after I brought this headcanon up to some friends @crazychimken12 found these younger pictures of the actors for Wilton and Devon
Now compare them to Garthe
I'm sorry, but there's no way Wilton is related to Garthe.
Devon Miles, you are the father.
