Zargerth

If I'm not mistaken, for a Dark monotype run you're allowed to use Froakie and Frogadier despite both of them being pure Water- only Greninja has the Dark typing. Following similar logic, you definitely could use Pupitar in your color-themed run despite it being different color than Tyranitar, if you're planning/allowed to use Tyranitar.

I did a Ground monotype run of the game some time ago. It wasn't my first run of Desolation, so I had some foreknowledge of what to expect. I specifically chose Ground type because I wanted to have options for my team from the get-go. When you do a monotype run as your first run, you can't really plan ahead and know what mons are available to you and when they'll be available. Ice types tend to be rare in the early portions of almost any Pokemon game, unfortunately. I wouldn't fault you for using Debug mode; at the end of the day, it's your run, your rules. It's almost a necessity when you take into account level caps and the scarcity of mons of your chosen type. If deviating from the "intended" way of playing makes the game more enjoyable for you, you should feel free to do so.

My guess is that if this is about Rain Dance, then that would count as a move that targets all Pokemon on the field and therefore bypassing Prankster-blocking.

For Singles that'd probably be Terra. I'm always caught off-guard by just how much damage that Garchomp Draco Meteor does. You can also get easily walled by her Palossand if you're not prepared. Aside from her and Ciel, I don't think I've had any other roadblock Gym Leaders in Singles in any of my playthroughs. As for Doubles, it's Charlotte. She can delete your entire team with relative ease, even if you have resists for her Fire-type attacks. While Shelly is tough, I've never found her to be a difficulty spike or a roadblock (assuming Hardcore or Redux doesn't count), and Hardy is more annoying than tough.

Just defeated E4 and Lin with this team. No Anna ending for me, but that's all right. Despite him not being on this final team, Bewear was a true MVP throughout the game.

How far in development is Phoenix Rising? Last I checked, there was just the demo that ended in the port town I can't remember the name of, where the museum theft happened.

I think having several Steel-types is fine. They'll be good to have given what Gyms there are left. Magnezone is one of the GOATs of Reborn, and personally I love Alakazam (even though it's not a Steel-type). Magic Guard is fantastic since it negates Life Orb damage, and mine had Psychic Terrain to not only further boost Psychic but also block priority moves like Shadow Sneak, Bullet Punch and Sucker Punch. I don't have much experience of using either Crobat or Goodra, but I find it a bit hard to believe Goodra would be a worthwhile addition to your roster at this point. I could see it working in one of the remaining Gyms due to it's resistances and high Special Defense (and ability to learn Sludge Wave) but I have my doubts about its long-term viability. On the other hand, Hawlucha at least on paper seems pretty good, especially if it has Unburden ability. You can use many of the Seeds to not only boost Hawlucha's stats (depending of the field effect), it also activates Unburden which doubles its Speed. That, combined with strong STAB options like High Jump Kick and Acrobatics, and in my opinion more useful typing given what Gyms remain, are all things to consider.

My understanding is that prior to Gen 7, move relearner could only teach your mons level up moves they had already learned. From Gen 7 onwards however move relearner can teach your mons level up moves they had not learned yet. For example, it should be possible for Silvally to learn Parting Shot in gen 7 through move relearner even if Silvally was not lvl 85.

My cursory glance at Bulbapedia reveals that there are differences between Gen 7 and Gen 8 Silvally. In Gen 7 Silvally learns Parting Shot at level 85, but that shouldn't matter since from Gen 7 onwards you should be able to teach any level up move.

I'm wrapping things up in Murmur Tower, but one thing eludes me. Any tips on how to get these two chests?

I'd say Discharge, unless you're going for something like Nasty Plot + Thunderbolt. You'll unlock the move tutor pretty soon anyhow iirc, so you can change to Thunderbolt later. There will be double battles as well as more battles on fields where Discharge increases in power. For what it's worth, that 30% paralysis chance can come in clutch. Of course, if you don't have good partner Pokemon to work with Discharge, then you might want to opt for Thunderbolt instead.

I'm inclined to say Girafarig, but fwiw it gets Baton Pass so maybe someone has used it already. As for Parasect, it has its uses because its one of the few (if not the only) Bug-type that can learn Wide Guard. Pretty useful move to have when you're doing a Bug monotype run and face against Charlotte.

Mightyena is a good choice, especially with Moxie ability. Then there's Linoone, but you'd have to level Zigzagoon all the way up to 20. Goldeen with Lightningrod as its ability is a decent pick as well. You could also grab yourself a Trubbish mainly for Toxic Spikes.

It's been a long time since I played Rejuv, but I'd go with Mr. Mime, because it learns Reflect and Light Screen early on, you can also teach it Flash (hint: Flash can change some Fields to your advantage) and for what it's worth it can get Filter as its ability. None of Mr. Mime's abilities are useless, but if you go with Technician, I believe Confusion is stronger (50BP x 1,5) than Psybeam (65BP) so be wary of that. Soundproof also has its uses, albeit more situational.