They appear to quite dry (not necessarily a good thing), so your biggest enemy is handling/moving them. The less they move, the longer they will last.

Depending on what is behind that paint, it might be gypsum that has dissolved out of the wall and through the paint. If that is the case, I would not try to hang any pictures on that wall any more.

Is Ellen Musk a new fragrance from Ms. DeGeneres?

Pine resin was also one of the ingredients used in the embalming process in Egypt due to its natural antimicrobial properties, so pine needles are naturally quite microbe resistant. IIRC, If you keep pine needles cool while drying, the chlorophyll can remain stable in which case the needles will stay green for an incredible amount of time.

6 years in an apparently relatively dry environment is hardly enough time for appreciable decay to occur. There are several reasons why the Atacama and Egyptian mummies can survive 1000s of years in the condition they do, and lack of moisture is one of the primary ones. My grandmother pressed flowers in books sometime in the 1940s. We discovered them a couple of years ago when we cleared out her house; they still had most of their original colour and were in remarkable condition.