Why would someone want to back out a plug and expose the threads in the barrel to the pressures and heat of the powder charge. Doesn't make any sense to me. But then a lot of things people do don't make sense.
I don't have a particularly thorough answer, except that the "backing out" is merely a slight crack open of the plug, to prevent freezing / bonding of the plug to fouling. The word was slightly. No threads are directly exposed by doing so.
The pressure spike in muzzleloading is momentary, and cannot be compared to sustained heat and pressure levels seen in the automobile industry for example. Several sidelocks (Thompson Hawken) have removeable breech plugs as well, though that is seldom discussed.
The pressure experienced directly by threads are very small in area compared to the breech plug face itself. While popular use of inlines may seem "new," there is about a twenty year history of heavy use. They date back to Pauley in 1808.
Few shooters manage more than 100 shots per day, some will not fire their guns that much in a year. With far less heat and pressure than from smokeless rounds, throat erosion is not a primary concern. The elongated snout area of many breech plugs takes the bulk of the momentary pressure spike upon firing. I know of no issues with thread stretching, but that does not mean it does not exist. The breechplug is an inexpensive, easily replaceable part-- and there is no particular issue with thread distortion from Knight, White, Thompson.
So, most breech plugs have threadless snouts that fit into the barrel, of varying lengths, and there are no directly exposed threads to the primary flame path. One of the shortest breech plugs is the White Rifle type, which is ordnance grade stainless steel (416 or variant-- not sure), hardened to Rockwell 50. I don't have the specifics for you, but the pressures applied
directly to threads are less than one might think.
The highest normal pressures in muzzleland are by the Savage ML-10 smokeless muzzleloader, using a carbon steel threaded breech plug with a very long snout. Those barreled actions have been tested to well over 80,000 PSI.
In a sealed action like your Encore, the pressure is contained to a great extent by the action, anyway-- just as it would be if you added a shotgun barrel to it. The Encore action is one of the strongest in muzzleloading land, as you would expect on an action that accepts a .375 H & H barrel.
