You're splitting hairs. The OP's question was why is 45% considered the softcap and why do some people build for far higher, and I answered in the most simple way possible. They didn't ask for a dissertation on how tohit, accuracy, and defense work.
45% is considered a soft cap because the base chance to hit of normal critters is 50%. Since there is always a minimum of 5% chance to hit, 45% defense brings you to a point where most critters are at the 5% minimum. But there are various critters throughout the game that have higher than base 50% chance to hit. Against those critters 45% defense isn't enough (especially on higher-difficulty task forces), which is why people build for more. Building for more than 45% also helps for when you get hit with defense debuffs, which are pretty common in the game. Most defense-based armor sets get some degree of defense debuff resistance, but it's not something you can get via IO sets.
Resistance caps vary by AT. Tanks and Brutes are the highest at 90%, HEATs and VEATS are 85%, and everyone else is 75%. You can build for more, but that only will come into play with resistance debuffs. Since your damage resistance values are also your damage debuff resistance values, in theory if you build to 100% or more it would effectively negate any resistance debuffs.
Light Lass from the Legion of Super-Heroes has gravity powers, the only hero I can think of off the top of my head.
Gravity for a Controller or Dominator doesn't synergize better with any one set over another, so really you can pair it with anything that looks interesting to you.
The amount of DDR and whether it's enhanceable is based upon what other kinds of damage mitigation a set gets. SR has none other than the minor amount of scaling damage resists. Energy and Ninjitsu have a heal, Ice/Invul/Stone have Hoarfrost/Dull Pain/Earth's Embrace, Shield gets moderate resistances and a damage debuff.
The PvP zones can be a total pain in the ass, especially if you run into someone of the other faction. There are a few badges that are insanely difficult to get to.
Just because you don't like it doesn't mean it's not true. The game has ALWAYS been balanced around SOs, even today, because that's what's easily and readily available to everyone at all levels with minimal financial investment. The majority of players don't have a meta endgame build or play hardmore endgame content. Believe it or not they use SOs and/or common IOs, even at level 50. IOs and Incarnates have ALWAYS been advertised as optional features, not required.
Being able to buff yourself isn't an exploit if the power permits it, especially since none of the buffs in question can be stacked by the same caster. There are plenty of support sets that get pets to buff off of. The easy solution is to just make the buffs PbAoE instead of targeted (like all the other team buffs that also affect the caster) so solo characters without pets don't need to worry about it.
The game is balanced around SOs, not IO sets + Incarnates. I think you'll find that Defenders and other support ATs are far from atrociously powerful with an SO (or even common IO) build.
Kheldians can be fun to play, but I agree they are a lot of work and a pain to build for what is ultimately a mediocre performer. The game and the ATs the forms are based upon have evolved, but they haven't.
Also no defense buffs IIRC.
The P2W vendor buffs also give valuable stats but they are prohibitively expensive after level 1. But it is one way a squishy can get full mez protection.
Make new characters with a powerset combo I've never done before and/or do a concept character, like a human-only PB, petless MM, melee-only Blaster, etc.
Sometimes I'll remake a character as a different AT, see if I like that version better or worse than the original, like Blaster to Sentinel, Tank to Brute, Scrapper to Stalker, etc.