Maskerade/WMG

WMGs for Discworld/Maskerade. Warning: Potential unmarked spoilers.

At the end of Maskerade, Walter Plinge drove Agnes away from the Opera on purpose.
At the end of Maskerade, the "new and improved" Walter Plinge seems to coldly reject the talented, smart, but fat and insecure Agnes Nitt in favor of the vapid, tone-deaf but beautiful and slender Christine because the latter has "star quality". While Plinge and his "Phantom" persona do seem to be different people, it always frustrated me to see the once-kindly Walter do that sort of thing. But in retrospect, it's possible that maybe Walter was well aware, or at least realized after his "transformation", that Agnes wasn't really enjoying her life at the Opera, being the Only Sane Man and therefore frequently confused and/or outraged by the behavior of her fellow performers. Therefore, it seems likely that Walter is discouraging Agnes because he knows that she'd be miserable in a Dysfunction Junction like the Ankh-Morpork Opera House. Sort of a platonic version of I Want My Beloved to Be Happy.
 * On the other hand, it's also true that the Ghost chose Christine over Agnes in the first place, on the grounds that "I can teach you to sing like her, but I cannot teach her to look like you" (but Christine hid under Agnes's bed when she heard the voice, and Agnes did her best Christine impression to get the lessons). The Ghost, unlike Walter, really is a bit of a git. On the third hand, if the "new" Walter is an amalgamation of his two personalities, it's possible he had both motivations at once.

The late Mr. Plinge was the Opera House Phantom.
Spoilers: He was the one to whom Mrs. Plinge referred by "he said he wanted someone to watch the opera" (emphasis Nigel Planer's, if it wasn;t in the hardcopy). This likely started back when the Opera House was comparatively new, likely around the time The Disk was built, and "singing-plays based on Dwarfish theater" would be even more of a risk of customer unavailability than the commoner plays. He always stayed in Box 8, even when the Opera House became popular amongst the rich of Ankh-Morpork. He wore the disguise so that the members of the Opera would feel better having a Mystery Patron than just the janitor watching them. .

Mr. Pounder the ratcatcher was reincarnated as Darktan from The Amazing Maurice and His Educated Rodents.
Mr. Pounder's new rodent incarnation would've been born in Ankh-Morpork, close enough to the University to scavenge its refuse (thus aquiring intelligence and a longer lifespan) just a few years before the Educated Rodents met Maurice and left town. Darktan's exceptional talents for identifying and disarming traps are appropriate for someone who, in his previous life, had won the Golden Mallet for the most rats caught for five years running. Having killed so many rats as a human, yet always respected them as worthy opponents, it's only fair for him to spend his rodent life teaching inexperienced young Trap Disposal Squad members how to elude ratcatchers' devices. And by helping to foil the pair of corrupt ratcatchers in Bad Blintz, he not only avenged their abuses of keekees and Changelings, but he also got to punish them for being a disgrace to his former profession.

Walter was named after the proverbial Walter Plinge.
Since his parents (or at least father) had been such fans of the opera, that is, making the joke and the name a sort of Zig-Zagging Trope.

Walter wrote the Disk's version of The Phantom of the Opera out of remorse.
When Nanny Ogg found the original version, her description gave off a Her Codename Was Mary Sue vibe. But the Phantom in the novel and play, is, if anything, more messed up than Walter. This is probably because after Walter had gotten used to his new identity, he felt like a bastard for letting the best voice he'd ever heard get away from him. So he digs out that silly old Self-Insert Fic and rewrites so that the dashing Ghost is a mentally unstable Woobie, Destroyer of Worlds and the most significant beauty of the heroine is her incredible voice.
 * That's adorable. I'm totally keeping this one as personal canon.

Christine is actually a Wrong Genre Savvy Bitch in Sheep's Clothing.
Agnes notes at one point that Christine is "remarkably clever in some specialized ways", and Christine is awfully short with Agnes after she blurts out her suspicions that Walter Plinge is the Phantom. So, what if she's just not as dumb as she lets on? In Christine's case, she knows she's in a story like that of The Phantom of the Opera, but like Malicia from The Amazing Maurice and His Educated Rodents she thinks she's the main character. All those times she was "accidentally" insulting Agnes and yanking the dogs' chain? All an act to mess with Agnes, of whose talent she's jealous, in yet another reversal of Christine and Carlotta from Phantom of the Opera!

As for the whole "can't sing a note" thing, she either has an exaggerated opinion of her ability to sing, or is just pretending to be dreadful so she can "miraculously" become a great singer (once she's frustrated her more musically gifted rival Agnes into quitting the opera, that is); it's been noted in other Discworld books that sometimes you have to be really good to appear to be really bad. This also means she was pretending not to notice that Agnes was "ghosting" her during the opera,.

As for why she hid from the voice of the Phantom behind the mirror, she just didn't want to get involved in any dangerously tragic romances with a deformed stranger, which is what she assumed would happen if she got involved with the Phantom because, again, she's Wrong Genre Savvy. Though this makes it look like Christine is throwing Agnes to the metaphorical wolves by letting her get involved with the Phantom instead...

Christine has elvish blood in her.
She's beautiful, and seems to capture the love and attention of everyone around her except Agnes, who ends up jealous of her good looks and annoyed with her self-centered vapidity; since Agnes is a witch, albeit a reluctant one, she's resistant to Christine's low-level glamour. And much like the Fair Folk, Christine has no musical talent whatsoever.
 * Discworld elves can sing well; it's playing music that they suck at.