A single entry non O is valid for 90 days from the date of issue. On entry you would get 90 days.
A multiple entry Non O is valid for a year from issue. On each entry you would get 90 days. You have to leave Thailand and return on or before the 90th day in order to get a new entry stamp.
Very few consulates will issue a multiple entry non O these days.
No not mandatory, but it may be your local office policy. You’ll need 15-21 days left on a valid stamp to apply so it may be necessary to get it anyway depending on your current stamp.
If you haven’t had the standard 30 day extension then most offices will make you get that first. Once that’s winding down you can apply for the 60 day visit Thai family visa. Beware though because you are “changing the reason” for an extension they may date it from the day you apply, so you’d loose any remaining days of your 30 day extension. Ask your office for their policy on this.