There is only one insect that gets into dry pine and that is the old house borer and it is seldom seen. So, you can be confident that you will not have any insect issues unless you store this wood adjacent to infected wood.
Your moisture meter is not working well, as it is virtually impossible to get under 3.5% MC with the conditions you had. The pin meter cannot register under 6.5% MC and the painless under 4.5% MC. On the other hand, even 5% MC is too dry for your ceiling, as in the summer weather, the wood will or would try to expand as much as 1-1/2% in size in the summer from th dry MC that you have now. Of course, the wood will dry out to 6% MC in the wintertime, so some shrinkage will occur. This normal movement must be considered when deciding the style (like tongue and groove) and the installation procedures...for example, the tongue must be long enough and only one edge is nailed so that the wood is free to expand and contract perhaps as much as 1% annually.
Note that very low MC also means a lot of chipping when machining, so 55% RH or even 58% RH is good now.
Are you close to Athens? I could let you borrow my meter.