Well, some of the openings we saw in the drydock scenes weren't hatches, but hull plates that hadn't been attached yet. The only hatches on the saucer that are indicated in the published plans are the two airlock hatches on the forward underside of the saucer, the small personnel lift hatch on top near the front of the ship (where the leads egressed to walk to V'Ger's core), and the main gangway hatch on the port rim of the saucer (which had a gangway connecting it to the drydock frame while it was in dock).
Right. I was just looking at the images showing these openings, and they're clearly outer hull plates that weren't fastened into place. Given that most of these areas were illuminated by spotlights from the dock, it's likely these were some of the final areas/systems of the ship being adjusted before launch.
All of these small details really speak to how rushed the Enterprise
launch was. We don't know precisely when the original launch and shakedown were supposed to be, but it's generally accepted that they were still several weeks away.