I get what you might be looking at, but I think you're reading it wrong. Just for sake of looking at things, I'll past the redline I think you're looking at here: -A Structural (Non voidal mage -3) modifier may only be applied once. This means, regardless of how many Voidstalkers or Obelisks are present, each only count toward the modifier once.
This is just saying that any non-voidal mage modifier can only be applied once. So you could apply an Obelisk and Voidstalker at the same time. However, you couldn't apply 2 Voidstalkers or 2 Obelisks at the same time. Any more voidstalkers than the first would just count as a -3. I don't believe it means that if you have one you can't have the other
This should probably read like this redline does: -A Mana Obelisk, at best, fortifies the manapool of mages within it’s range, reducing their voidal exhaustion by half.
The difference is that it makes their spells cost half rather than them having 1.5x the mana pool. I don't think voidsalkers were meant to have 2x mana pools. Squak recently talked about it in the Story Discord sharing that they're meant to have the effect of Obelisks, which is the half costed spells
Personally, I don't think either of these two things need to be changed. You already technically have to work with someone if you're doing Arcanium forging, everyone forgets you need a designated Smith. For the mana pool thing, I think it's fine since it's the staple thing of Voidstalkers.
For Wither/Invasion, I don't think it's needed to have the line about allowing multiple Voidstalkers to cast wither/invasion on people in combat. I think the rule Wither has currently could be transferred to Invasion though instead where a Voidstalker can only Wither the same person once per combat. I can't remember if that has already been given to Invasion though. I also think that if we're going to make it cost a T5 spells worth of mana, it should also deal 5 points to Ward Shields
Those are my issues, but I do like quite a few of the things you've given. The 4th slot needing to be voidal is really fun, same with the Anti-Magic part. It's always made sense to me that all anti-magic should do something like that to a Voidstalker. I've never used Arcane Replenishment in my life, so I have no feelings about that. Finally, for Wither/Invasion, I think the tells are important. More specifically, I think it should require you to place aura around your target rather than just around your hands so the target knows they're gunna get hit with something, maybe add this as an Invasion redline too. I also think the stable connection part of the Wither change is nice, though I wish it was able to reduce scions back to normal voidal mage levels due to their weird magic seals and being vulnerable to that sort of thing. Might just be me hating scions though