Even in this digital age, people are used to being still for getting a picture taken. I find it helps if you tell them how long they need to stay still.
One second or even a few seconds is generally not a problem, even for kids. For example, look at some of the portraits done on wetplate, 1 or 2 seconds is a short exposure in that realm.

DoF doesn't need to be a big problem either, mostly it's a problem of the subject going forward or back in relation to the camera, most people will move more side to side than fore and aft. Other than that it is what it is, and generally a subject's nose and ears won't both be in focus, if either are. If the basic plane of the face (eyes, cheeks, mouth) are, then you're golden.

Large Format lenses don't have all that much less DoF than a portrait length lens for cameras you have, if you're looking at 4x5.

There are mirror viewing gadgets that invert the image, most of them are specific to one camera or another, but would tend to be adaptable. It's easier though, to get used to composing upside down. It's easier than you might think.