One opinion:
1. I wouldn't worry about it. If it's keeping time and not exhibiting problems it's likely to be OK. If not then it will need to go in for service. THat's the chance you take. I've bought several complications double sealed without a problem.
2. A double sealed watch is worth more to a collector assuming the piece is collectable in the first place. A double sealed watch has been untouched from the factory meaning everything is perfect brand new - no blemishes, polishing, abuse, strap changes, etc. Having the price tag, etc. is even better but again this is only important if the watch is collectable. Not all Pateks are collectable.
3. I think this answer depends on the oil inside the watch. If the watch has been kept in a climate where the oil can evaporate then the parts may be dry. Again, this is the chance you take but IMO if the price is right it's worth it for a collectable piece.
A still-sealed watch would actually be less attractive to me. I want to ensure that it's not been damaged in any way and that all the complications work. Small issues such as a date not centering properly can easily be spotted without a plastic bag in the way.
I am not however a big-time collector so others more informed than me may well see it differently.