Sorry I just have to point out where I disagree.
Oak and Erebus have minions which do not scale as much as Sedna nor QoT.
At level 10 minions have base:
Spirits : 840 HP, 64 DMG
Nightwalkers : 1130 HP, 38 DMG
Yetis : 1710 HP, 51 DMG
Shamblers : 675 HP, 30 DMG
In addition to this Yetis have Heavy Armour (same as Giants). Shamblers have 2.5 Radius damage.
Sedna and QoT can also buff their minions with skills.
Qot has compost which can add 14 DMG and 360 HP to base and Entourage which adds 750 and 30 DMG.
Sedna has Magnificant Presence +200 HP and +15% Attack speed and Healing Wind 2, which gives them +24 HP/sec.
So Yetis can be 1910 HP, with 24 HPs regen and +15% attack speed.
Shamblers can be 1785 HP with 74 DMG.
Shamblers are also ranged, meaning they do not suffer from pathing problems.
Yes, they are still weaker than 10 minions combined, however, they are far sturdier and less vulnerable to AoE than Oak or Erebus.
Decrease QoT's skill dependency on shamblers, and make Sedna's synergize with her play more.
This sentence makes no sense, you are saying that one DG is gimped because they have to rely on minions too much, yet you say that another DG needs to rely on their minions more. You would just end up doing the same, but the other way around.
I would argue that QoT actually has great minions and a great supporting skill tree. The problem is that most player don't know how to use that and write it off. I have played minion based QoT a lot and it is a lot of fun and most probably her most powerful build.
Sedna is indeed gimped with her minions, however, this is mainly due to the fact that Yetis are dam slow and stupid. Pumped up they are great building smashers, but they are poor at taking down DGs and spend more time hitting grunts.
A big factor you haven't mentioned is that both Erebus and Oak have a snare, this improves their minions a great deal, as DGs will have trouble running away. Part of the problem of Sedna and QoT is that they cannot snare and so a DG can easily get away, especially as their minions are slower.
Also, Spirits have no pathing problems, furthering their ability to chase down DGs, Nightcrawlers are smaller and have less pathing issues.
I don't think QoT needs to be changed apart from a slight decrease in the casting of shamblers, this is killer, especially at early levels. I would also possibly add that she should have 6 shamblers instead of 4.
Sedna needs her Yetis to either be ranged or the ability to go through creeps. Possibly change wild swings so that it knocks creeps out of the way, also make it a lower end skill, maybe after counter healing. The mana cost needs to be dramatically reduced for Yetis too. She has plenty of skills that synergize with Yetis, she doesn't need anything else, that is what makes QoT unique. I would also buff their HP and DMG at later levels up a little more.
I don't disagree with most of your analysis, but I do disagree with the end conclusion. QoT is an interesting DG to play, with great minion based skills. Sedna has more immediately powerful assassin build type skills, she is let down by having immensely stupid minions.