RE: Wet powder, no buck!!
That sucks, hopefully you'll get a second chance. Do you cover the Muzzle end of the Barrel? If not, do!! I use electrical tape but you can use finger cots, condomn, plastic wrap...anything that keeps the moisture from entering this way. Just make sure it is on the outside of the barrel no the inside and you'll be good. If you have an open breech you can drape a wool sock lined with a plastic bag over the action to help waterproof this end of things. Then when the time comes remove the sock and take aim.
Yes when it is wet you should dump the charge, clean, re-oil, swab the excess oil out of the bore and re-charge with fresh/dry powder.
The other option is hunt in a shelter blind...LOL!
Good luck to you.