I gotta ask - where are your son and his fiancee in all this? They want to get married in 6 weeks so most of this should fall on their shoulders, not yours.
They are both heavily involved but have asked DH & I and STB DIL's parents to help, which we are all very willing to do.
I'm getting some great ideas here, thank you!
Just to address this point--it's great that you guys are willing to help out, but I would just be super-clear with the HC about what that means. I mean, that sentence--"have asked DH & I and STB DIL's parents to help, which we are all very willing to do"--could simply be referring to handing over a check, while the HC make all the arrangements.
I think what I would do to help, is do the tedious things that the HC have already decided on. Maybe I would buy and cook the food they told me to, in the amounts they wanted. Or after they picked out decorations, I would work alongside them to decorate the space. I could print the invitations and envelopes after they already vetted the guest list and addresses. I could tie up M&M's inside little circles of tulle after they showed me what they wanted.
I don't think there's anything wrong with taking an even more active role, but it does seem to be stressing you out, and I think really there's no need for that. The HC are adults and made this decision to get married quickly and while of course you don't want to leave them out there to dangle, it would be okay to let them run the show, and just be the helper. They want hay bales? Then they figure out where to get them and how to get them to the location, and maybe you agree to meet the truck. And you could say, "Hay bales are prickly, what are you going to put down over them for seating?" in case they haven't thought of that, but then let them figure it out. Then you could offer to go buy/pick up the thing they decided on.
Maybe they won't get everything they envision--I don't think a hay bale sounds very comfortable to sit on, personally. An HC might not get everything they envision no matter how much time they have to plan. That's life. I just get the vibe that maybe you will stress yourself out too much, trying to make things perfect for them, and really, perfect should not be their goal or expectation, and it definitely shouldn't be on you to deliver it.
Just some thoughts.
I think a nice wedding in six weeks is totally doable, but you don't want to look back and think, "That was the worst six weeks of my life!"