This is slightly opinionated but mostly well researched as well. I tried my best to answer everything as best I could!

What if split-normal damage was removed from the sealed sword and brandish line?

Damage dealt = attack multiplier * damage - defense

This is applied twice with brandishes, which means they have to overcome extra defense. If you moved their normal to their special they would be even more powerful. If you nerfed them such that they did the same damage at 0 boost they would get half as much bonus from damage up bonuses.

What if universal DMG bonus was nerfed to boost at max as much as at high?

This would be somewhat tricky to code into the game as all these bonuses stack and are then calculated. But mostly it would make a lot of black kat and chaos users rather sad... but overall I don't think it would change anything much since MSI and CTR are still very powerful stats that aren't on other armors. Rather than this, perhaps we could make other armors worth using? Black kat and chaos are fun. Let people have fun.

What if DMG bonus vs "monster family" could be stacked with universal DMG bonus?

It can. If I have black kat and virulisk on then I get damage ultra (+5) vs slimes and damage high (+3) vs everything else. If you mean past max then it would be nice but not really worthwhile for most things. Damage bonus is 4% so max damage is 1.24x damage. Following this...
Med = 6.4% damage increase over existing = .08 / 1.24
High = 9.6%
V. High = 12.9%

Some items would be quite nice with this when it procs, but overall it wouldn't be nearly as strong as having say, charge time reduction or attack speed as your UV... plus it would make people think twice about rerolling a beast vh... which you want but GH probably doesn't. We spend A LOT on those asi/ctr uvs.

What if the Troika line was buffed to deal as much as the GF/DA against their respective neutral targets, as they are now?

It still wouldn't be as strong, because well, see question 1. But you could balance it so that it does more base damage, such that at max damage bonus they're equal, or such that troika has normal and normal damage. Normal/normal would make it gain even higher damage than other things... and I'd like to see that.

What if the charge of the Troika line was buffed to deal as much as the Acheron's charge against neutral targets, as it is now ?
And what if the charge's radius was increased?

Well again with the split damage thing. The math doesn't allow it to deal the same as acheron's charge because of damage bonuses. If it was stronger however I could see the sword being a lot more viable. That being said in my opinion it should be faster rather than just stronger. Balancing speed and power is no trivial task.

What if the Sudaruska and Triglav line had a chance to deal their respective status on normal hits?

Triglav does, on its second hit. Sudaruska does not however. Honestly I'd love to see this changed to like a good 100% chance of status on hit. This would change how the swords are played drastically, and would likely make them quite viable compared to other swords.

What if the Chromalisk gear would make you invisible after standing still for a while?

Then it would be awesome and balanced with black kat.

What if the WHB had a chance to interrupt mobs after its 3 swing?(As it is slower than the DVS)

So first off it does have more interrupt. You probably know this. That being said, interrupt is kind of mysterious as far as I know. Like it's based on damage and some other factors... I'm sure turning up the interrupt would be nice? I don't think you can just have the third hit do interrupt like crazy and always work though... at least without lots of coding work.

Secondly the big issue with WHB and DVS is that they're single target weapons that are out shone by brandishes, blitz needles, alchemers, etc. For them to be good they need to be able to achieve damage that's worthwhile AND have some areas with enough variety mobs that require killing with single target dps to warrant bringing a normal damage option.

Personally I want them to get an attack orb for 30 seconds (+30% damage!) whenever they do their charge and hit something.

What if all weapons with innate DMG vs "monster family" high were buffed to VH or ultra?

It would certainly help their case for newer players, but the endgame is very much centered around black kat and chaos, and even features mobs that are family-less like vanaduke and crusaders.

What if the Leviathan could inflict stun on its charge?

I have a celestial saber and the stun on charge is mildly helpful but honestly not that big of a deal. It's kind of neat I guess? Occasionally it will do something epic (stun a mender + get them out of the crowd) but usually it's just kind of there and doesn't help/hurt anything. A buff, but a very small buff.

If the stun were more reliable then I could see the sword going places though.

What if the CIV was buffed to match the Leviathan?

I'm assuming that you want levi with stun and CIV with damage vs undead ultra or something. That would be fine, but CIV has warmaster rocket hammer and even blitz noodle to compete with for the job of killing deadnauts and howlitzers. It'd be more balanced though, I'd say. Not worth crafting, but not as much of a "trap weapon" either.