.......Can it be closed again inflight after it was opened?
The original 747 Classic ( 100/200 series ) was designed to take a periscopic sextant as per the 707 - and other a/c of the period - for use by a navigator. By the time the aircraft entered service, INS had proved itself so no specialist navigator was ever carried, but the pressure lock hatch that allowed the sextant to poke through the roof of the aircraft was retained as a 'smoke removal port'
The sextant was inserted into the bottom of the hatch, making a pressurisation seal, then the roof flap was opened with a handle on the side of the assembly, and I can assure you that there were times when I thought that the pressurisation force would extract me with it ! certainly the sextant had to be held down firmly and only allowed to slowly ride up further, into place. After use the procedure was reversed, the sextant being pulled down far enough for the hatch to be closed again.
Certainly no modification to the sextant hatch was ever made to any 747 I flew, through to the - 300, so in those aircraft the short answer to your question is ..
Yes !
I know nothing about the -400, but I believe that there is no change ?