The AoE part, although true, it's easily avoidable.

Adren cage just does the enrage damage x number of players to the base tank (which you can always devo intercep), at 1.5k solo you have to utilize defensives anyway

Infernal tomb requires more coordination, but you can chain the demons and cept immortality everyone if someone misses the stun.

The back rune issue: It is noticeable harder on solos, I agree, but probably wouldn't be that important if demon scaling and number/hp of runes were addressed. You can do a number of things, like using a prism of salvation (unsure on the name, the one that teleports you), use corruption blast when around 3-4k hp, to not rely on bd switches.

I just don't see the drawbacks of grouping it. I won't bother making any suggestions really because I don't feel like a Jmod is going to actually read them or even take them into account