Series 50? model 53 plow

i) "Not flipping the plow onto yourself" - not sure what you mean here. Are you worried the traction booster operation might somehow cause grief and rolling the tractor? You don't have to worry about that. At least, not as it relates to your plow/tractor setup. On our really steep side hills, plowing is one of our most dangerous tasks, because driving with the downhill wheel in the dead furrow means you're that much closer to tipping. I've gone up on two wheels in the air a few times, but haven't tipped/rolled yet. But this is nothing to do with the tractor/plow and 99% of people would never have this problem. It's just the way our land is.

ii) RH lever is the traction booster lever. When you say, 'splitting it open", do you mean splitting the tractor? Or just disassembling the valve(s)? You certainly don't have to split the tractor (or, at least, not unless something pretty drastic is wrong). It's likely just a stuck traction booster spool/cam. Some penetrating oil, tapping the spool when removing the rear valve cap, and working it a bit is usually all that's required. Other folks on here willl be able to help you better - it's been years since I've unstuck one. But it certainly wasn't a big deal. This isn't a 'speed control' lever. It's to adjust the amount of the plow weight the rear end of the tractor carries to help it power through tough spots. Your wheel RPM stays the same, it just helps it not slip so much. I could get into a long explanation here, but do some googling of ''Traction Booster' and 'Draft Control', and you'll find a lot of videos explaining it. Nothing to do with speed (or at least, not 'directly' relating to speed). All to do with slip.

iii) Post some pics of the plow and the rear-end/coupler of your tractor and folks here will be able to guide you in the right direction. I'll caution you, however, that a three-furrow plow may be one heck of a load for a D15. That completely depends on soil type and terrain, of course. I've seen several videos of D17's pulling four-bottom plows just dandy. But always in clean stubble on loose, black topsoil. On tough land and/or sod,  it's a different story. I have a 180 which, seems to have quite a bit of jam - even for a 65 HP tractor (it doesn't feel anything running a 4X5 round baler up and down our hills). But a 3X16 plow in sod and our soil will bring it to its knees. First gear 90% of the time, and even then you can spend a fair amount of time with your front end in the air, just trying to plow 6" deep. You may therefore find you want to drop a bottom from your three-furrow for your D15 to be comfortable. Not worth worrying about immediately, but keep it in mind as something you may have to do.