Breechplugs are made of Stainless Steel, so they need no lube to keep em from Rusting, store and always keep them Clean and DRY.
I have Never shot off a Primer befor I loaded My ML, there's No Oil or Moisture in it (mine anyway) so there's no need to.
Some like to shoot there ML on a Fouled Barrel, they say it's More Accurate, just think as your at the rang and shooting your ML after the first shot it always has some amount of Fouling in it even after a few swabs, so most of all of your shots through your ML will be with some kind of fouling in it so it only makes sence to have some fouling in it on your next shot at a Deer so the Firing a Primer though it befor loading it will provide enough fouling so your first shot will be where most of all your other shots at the Range were on the Target, Hope this makes sence.
I got LUCKY, with My ACCURA I can shoot it on a Clean Barrel and have the first shot go in the same place as my shots from the Barrel when it's Fouled. Now I dont mind a Fouled Barrel, if I shoot a Deer and reload I'll leave My ML the way it is, as I know it'll be Right on the Money for My next Shot. And I dont have to worry about the Fouled Dirty Barrel when I may leave it loaded for smetimes more than a week or so as It's a SS Barrel and I use Montana Extreme Bore Conditioner, and after a week or more and then shot and cleaned on the Dirty Barrel it cleans up Fast and is as Clean as New. You'll have to get to know your ML and see if the First Shot on a Clean Barrel will go in the same place as a shot on a Fouled Barrel. If your first shot is off then I'd say Shoot a Primer through your ML befor Loading for your next Hunt.
I use TC T17 Foaming Bore Cleaner, only because they had a Bunch of it left over after the Season at Wal-Mart and I bought 4 Cans of it. It works OK, No Ill effects from using it, why what have you heard????? I mostly just use Dish Soap and Water to Clean My ML Barrels, followed By a Few Patches of TC No 13 Bore Cleaner followed by a patch or two of Montana Etreme Bore Conditioner, qait a while, a couple of Dry Patches then Load.
I use TC BP Grease, or Bore Butter on the Threads and I have even used Teflon Tape. Depending on how much (Blow Back) you have the Teflon Tape does the Best Job to Prevent this, is Blow By is Minimal the Grease will do the Job.
To grease the Threads on the inside of the ML Barrel, I first make sure there Clean and use a Bore Brush to Clean them Good, then make sure as your Barrel is already Dry and Oiled, then I apply the BP Grease to the Threads of the BP, I serew in the BP, then Un Screw it, then back in again and out a few times to make sure the inside barrel threads are well lubed, then I just (Hand) snug the BP and Im good.
I dont have a TC Breech Plug with the O-Ring, I would'nt worry about your's wearing out right away but getting a spare would'nt hurt. My Buddy has a TC Triumph and His O-Ring has lasted so far after a couple of Seasons. But If it was Me I'd get one, I always have a Spare Brand New BP, I've never needed it but it's good to have when Murphy Shows Up.
Best of Luck to You!