Is it appropriate/wise to include personal/family reasons in a "Justification for Choice of Host" on a fellowship application?
I've been involved in a few searches where this issue has come up. Sometimes it helps the candidate, sometimes not so much.

When it helps: In trying to decide whether to bring a highly competitive candidate to campus for an interview. No one wants to go to the trouble and expense of an on-campus interview for a candidates who isn't going to accept an offer. Knowing that a candidate has compelling personal reasons for wanting to live in the area makes it much more likely that a seemingly over-qualified candidate will be taken seriously. 

When it hurts: Any time it looks like location is the primary reason the candidate wants the position. It's great that folks want to live and work and retire in our town; it's not great if they're willing to take a job they don't particularly want in order to accomplish that. It's even worse if they aren't able to speak to the specifics of their "fit" with the institution much beyond "I really want to work here".

In your particular case, I'm not sure that there's any upside of sharing this information. Your reviewers are already going to know that out of all the host universities you could possibly choose in the world, this is the one you prefer. What they don't know is why this is your first choice. If they learn that it is due to your spouse's location, they may wonder what would happen if your partner gets a different job, or your relationship hits a snag. Those who sympathize with the host institution may also feel that it deserves someone who loves it for itself, so to speak. It will be better for you if they believe simply that it is because you feel that this is the place where you can do your most brilliant work—which is probably true, as it's hard to be brilliant when your home life has been turned upside down—for a variety of reasons including your "standard points". At most you might want to say that you are familiar with the institution, and feel comfortable there.

As far as signalling why you have an employment gap—are you submitting letters of recommendation? If so, perhaps one of your references could speak to this issue. That was how I handled my own parenting-related employment gap when I returned to the workforce.