I agree with the general sentiment of OP's post, but I disagree in some details.
Yes, the allies are mostly useless. With the CM for normal mobs I have seen them dead before the escort event even started, too. Even if not, they die one-hit at the first trash mob. For non-CM the Seraph Watchknights are quite strong and can hold positions on their own, especially in Gendarran Fields and Lake Doric. During escort events they push mobs out of my attacks, though, which is highly annoying.
I think it is a bug, that Kasmeer doesn't help. In Metrica I need to pull the boss a few steps into Braham and Taimi's range, but that is not a big problem. In Gendarran I just ignore the NPCs, run ahead and leave them with their mobs. And in the other instances the story NPCs at the very least tank some trash mobs for me.
The scaling for the first three instances has been fixed with the chapter two patch. Before I could activate the timer and the stronger boss and get my 2/3 gold chest solo. Harder trash mobs were near impossible to kill and dealt way to much damage. Post patch I can quickly kill them before they are a problem. For the four new instances I have a similar experience. Scaling would be OK. But I don't see how I should be able to do them within their time limit, though.
I think it is ok, that CMs are not available in public instances. If they could be activated I can imagine players disagreeing whether they should be enabled and/or use them for grieving. They still could in private groups, but that's what block lists are for. ;)
About the achievement list: Top Dragon Responder: Metrica, Gandarran Fields and Brisban: no longer impossible solo. I know, because I did. Without a fancy super special DRM build. The other four: I don't see how they are possible, but I have not given up yet. Shooting Star and Killer Waves were easy, I did them in my first CM try. For No More Sacrifices I needed to adapt to have more CC, but still easy on second try. (The EMP from Drizzlewood is a common tip, I just changed one skill and used other weapons.) I wouldn't call the weapon collections "impossible", but I agree that throwing hundreds if not thousands of gold at them is not an option.
According to your screenshots you play as engineer. I don't play one, so I cannot give you tips for that specifically. I play DRMs with my thief on an open world build with berserker's gear. The trait Invigorating Precision (crits heal me) is great for this type of content. I play the pre-events with Daredevil and staff. Staff 5 is great AoE damage against the many small destroyers. It also works well for the Metrica boss. For the Brisban boss I switch from Daredevil to Deadeye and use a rifle. The side where the destroyer comes from is higher, so when I stand, or rather: kneel, there the shock waves pass below me through the terrain. I can completely ignore them. Rifle 2 pierces and takes care of the adds and CC. The boss hardly ever runs to me, and even if it quickly jumps back to the bridge. For the rifts phase I switch to melee weapons and then back to rifle when the boss reappears. This fight is almost boring even with CM. Similar with Gendarran Fields. I stay out of range of the tornados, the damage from the fire balls is easily healed by the trait. In the phases when the boss disappears I use mounts to go on higher ground, starting with the tent next to the arena. You only need to kill the rifts, not the mobs around them. The mobs can't reach you up there, so they don't attack back.
I'm pretty sure there are other classes and other builds that can beat these CMs solo, with a similar or different strategy.
For the four new CMs I haven't found a solution, yet. I can kill all the trash mobs and bosses with my thief, even with CMs, but not (yet?) within the time limit. If I don't find a way, I won't get this meta achievement either. But it wouldn't be the first. For me IBS means mostly unreachable meta achievements anyway. :(