Warden tank can be a bit tricky in this situation bc
a) blue RK is hugely reliant on pre-hotting a tank (especially warden with no real panic skill) and
b) warden lacks of utility for certain fights (like this one).
If RK had a tiny bit of experience with warden tanks he'd at least place his -inc dmg stone or even add Raid Bubble for the first few seconds.
What I'd suggest to do is after pre buffing (probably with conviction as last one to benefit from HoT-Agro (at least a bit)) do Goad to open up Battle Memory and do Surety of Death. Then Pre-Build Brink of Victory and wait for the dwarfs to enter. When the fight begins you instantly hit Brink and after dwarfs line up you fire off battle memory with SoD. That should easily generate enough agro to hold mobs against any healer and most DDs for at least 5-10s. Ideally you'd have a Champ and wait for big ching-ching until you hit DC. It's crucial to hit Brink perfectly timed at that point where mobs become active, because, as you stated correctly, mobs don't care about aggro switch while executing their skills. There are some fights in this game where the first 10-20s are absolute matchmakers. Like in Anvil ID2. If you figure a way to make it through these first seconds it'll only get easier.
Any single boss scenario I'd simply start with warning shot. After force taunt runs out chain it with Defiant Challenge. Depending on the fight (like certain add mechanics) I'd follow with a second warning shot. When doing so and still lose the boss your DDs are probably pretty trash and you're free to flame them publicly.
Warden - Nicl - lvl130
LM - Telperinor - lvl130
Burglar - Nicsa - lvl130
RK - Telpinquar - lvl130