For street photography a good technique is zone focusing. For example F8 on a 35mm you can pre-set your focus to 8 ft which means that everything from about 5-1/2ft to 15ft away will be in focus. No waiting for your auto-focus to catch the subject, as long as they are withing the zone, it will be in focus.

Here is a site that explains this technique in better detail

https://www.ilfordphoto.com/zone-focusing/

That language means you can’t make actual porn with it. I’ve worked on a show where we used stock images for fake girlie magazine covers. Have your legal department look over the TOS and sign off on you using them

Unless they were famous, you probably won’t get much money from the photos/negatives. That being said I personally think that a great way to create some sort of legacy for them would be to release them under a Creative Commons license. This way people who find the work useful can utilize the images and your FIL’s photography will live on that way