You're starting to slip back into "it wasn't in Guild Wars 1, so it doesn't count".
The unfortunate truth is that Guild Wars 1 (and, for that matter, Guild Wars 2) gives very sparse details even on the regions that we interacted with directly, let alone those that were basically background like Orr was. What we know about the fall of Orr from Guild Wars 1 was basically "the Orrian army marched out to fight the Charr, lost, and the Vizier called down the Cataclysm in what was believed at the time to be a last-ditch effort to protect Orr." There's basically no mention of any details beyond that. Guild Wars 2 has filled in some of those details. There's no contradiction here because there wasn't anything to contradict.
The Cauldron is a weapon (or, rather, they are). It is used to cause harm to the enemy. In the Searing, it was used as a strategic weapon to destroy Ascalon. In Guild Wars 2, it is used more tactically (in one case by choice, in another, because the cauldron in question was depowered). Nothing in Guild Wars 1 is contradicted because, again, Guild Wars 1 gave no details on how the Cauldrons worked apart from "perform a ritual and fiery crystals rain from the sky".
You're trying to base an argument over things that Guild Wars 1 left open - and it was policy right from the beginning to leave things open to be filled in later. (See, for instance, the references to Palawa Joko.)
The memory from the Cathedral of Silence story step appears to be after the defeat of the Orrian army, but before the Cataclysm. So one possibility is that they found out the hard way that the Searing could be repeated when their army got nuked. Another possibility... well, if we go back to earlier in this thread, I've already addressed that:
draxynnic, on 17 September 2015 - 12:31 PM, said:
In the case of the charr army, they'd seen what the Searing had done to Ascalon and weren't willing to risk that happening to Orr as well, so they marched out to intercept...
If you know the enemy is carrying a limited-range nuke with them, going out to intercept means that you at least have a chance of stopping them before the nuke hits the city and people you're trying to protect.
Regarding control: One possibility is that the Titans neither needed nor wanted the Flame Legion shamans to actually understand what they were doing, and just gave them enough information to use it as a strategic nuke without giving them the instructions for the tactical nuke option. Remember, the titans had their own agenda in which they didn't care one bit for the charr except in their usefulness as tools. Another possibility is that the Pact is simply showing its hubris - the GW1-era shamans may have known quite well how to use the Cauldron in a more targeted sense but chose not to (after all, from Pyre's dialogue, the 'scorched earth' result was part of their strategy).
Regarding how it's powered: My impression is that it IS a battery. Power comes from somewhere else (possibly the sleeping Kralkatorrik, although this was never fully confirmed), is stored in the Cauldron, and is released in the form of crystalline meteorites. That's why the Iron Marches Cauldron is only able to Sear a single village.
It's explicitly stated that a) the Pact sorcerers have more control than the Flame Legion, and that the Cauldron in Iron Marches is drained of power.
In the Cathedral of Silence step, the Keeper of the Shrine mentions that the charr plan to sear Orr as they did Ascalon, indicating knowledge on the part of the Orrians that the charr were bringing cauldrons and what they were for. I think there are other references, but that's the easiest one to track down. Lack of mentions of this in GW1 is likely due to no living Orrian surviving to tell the story afterwards. There isn't even a contradiction here, as the Guild Wars 1 lore is silent on the matter and this is simply filling in a detail.
And in regard to conflicts between GW1 and GW2 lore generally,,,
The ArenaNet moderators may have tolerated "I troll because I care", but I will not. This forum is for discussing Guild Wars 2 lore, which has been established to overturn some things that our characters believed in Guild Wars 1. Any further argument on the basis that Guild Wars 2 lore is invalid because it is not supported by Guild Wars 1 lore will be considered to be not contributing to the discussion and treated accordingly.
1) You have someone along who's studied them, and a large part of the following story step is defending them while they perform the ritual.
2) Zhaitan's forces moved the Cauldron in preparation for using it against the Pact.
3) The Cauldron can be tweaked. Furthermore, each of the cauldrons has a limited amount of power in them - the charr tactic in the Searing was to blow it all in one pop. Smaller effects are, however, possible if the user is more restrained or if the cauldron doesn't have much power left (there's an event in southern Iron Marches which demonstrates this if the Flame Legion aren't stopped from using the better-known cauldron that's still just north of the Wall).
4) All witnesses to it DID die in the Cataclysm. There are, however, various records of it in Orr from just before the Cataclysm.