When they were operating as a United Express carrier rather than a codeshare carrier, Silver did have PreCheck. I got it at BKW, a tiny airport in WV. All it meant is that I didn't have to take my shoes off, etc., vs. true PreCheck, but it did work. You may need to get Silver to add your trusted traveler # to your reservation, or it may convey over from UA's system if you booked it through UA. (If you get Pre off of status alone, without separate Global Entry / Nexus / Trusted Traveler enrollment, you're right to fear you won't get it.)

Check-in online is almost certainly not going to work via UA's website, unfortunately, for those FL flights since they're a codeshare now instead of UAX carrier. You may be able to use Silver's to do so; I've typically had good luck checking in online via the operating carrier's website for codeshares (assuming Silver does so). You'll need the Silver PNR, which you can call UA to get, or can get off of the reservation on United.com (it will list as "additional confirmation #s" or something similar on the reservation).