I finally ended up doing both the scraper and the pressure washer. I did the scraping first to remove all the flaking and loose paint. This part did some damage as I expected, which I later "repaired" with cement mix. The damage was mostly due to an electrical conduit that has rusted over the years. The garage is about 52-53 years old. Then I finished it off with the pressure washer which removed a lot more paint than I expected and left bare concrete in most of the ceiling. However, it *did not* cause any additional damage. All damage that could have resulted from the whole thing was from the scraping. Still have to paint but the hard part is done already.
The slab is not structurally compromised as there are no major cracks on the outside surface and it never leaked even with the ponding as I said before. At least not at this moment. Perhaps in 20 more years things will be different.