Hello my beautiful friend! I was in your shoes one year ago and am happy to report plural acceptances/waitlist offers! I am a German degree holder living in Ohio.
Your previous academic and professional success is extremely relevant. With those stats, you stand a chance. My understanding is SLPAs are on the decline, but check your local laws. My response to your concerns below:
(1) Is it necessary to do SLP-related coursework prior to applying? (From what I've read elsewhere, there are programs which claim to accept those without relevant coursework, but given how competitive admissions can be, it's generally a good idea to take SLP courses prior to applying. Is this accurate?)
Almost 100% necessary. Here's the thing, to get accepted you have to be exceptionally well qualified. Not just qualified. Exceptionally well qualified. Unless you are the Dean's child or have some amazing connection, you have to play the numbers game and get the odds in your favor. 500+ applicants for 25 spots. Your background already stands out as being related-ish and impressive. If you complete almost all, like, don't leave any more than 1 maybe 2 unfinished courses that you have a ready plan to complete prior to admission, admissions committees WILL give you a second glance if nothing else.
(2) Does it matter whether one completes an "official" post-bacc program vs. simply taking relevant courses?*
NO! End of discussion. "Official" programs are just marketed as such. #2 shouldn't be your most scary concern. #3 should be.
(3) Although there are many commonalities, different SLP programs have different course prerequisites. Does this mean that a given post-bacc program or set of courses could be adequate for some schools, but would disqualify me from being admitted to others?
This part drove me NUTS! I ended up taking 17(!) pre-requisite courses in 12 months to account for the commonalities as well as pre-req to the pre-req (think, of a math class pre-req to get into a physics class pre-req kind of thing) and also took a few based on the schools' requirements where I would be applying. Even if you complete a second BA/BS in CSD it still could be that your degree lacks a particular course that a grad program demands you have either prior to entrance. One school requires ASL, another doesn't. Better take ASL. One school requires research methods, another doesn't. Better take research methods. You get the picture. I advise making a spreadsheet and ticking off courses that give you the most bang for your buck, but absolutely do not neglect one-off courses that your desired/potentially desired grad program wants. This frustration will prepare you for the real world of working as an SLP anyway; some states have very similar yet slightly difference licensing requirements, and if you don't jump through the right hoops, no license for you.
(4) Any insight into extended three-year MA/MS programs which incorporate pre-reqs into the degree itself?
West Virginia University accepts you into the graduate school while you can complete a year of pre-reqs. At this point in the year, unless you already applied for this type of program, I believe you'll be waiting until next round. These programs seem convenient, but there are other methods if needed. I believe maybe Dayton or some school around that area in Ohio seemed to be the most accommodating to post-bacc kids like us (even though I did not personally use their program and cannot vouch for them--just know they market themselves as post-bacc friendly).