Yeah...in the same sense than a man carrying an armed nuclear warhead is more powerful than the city police force. It's power, all right, but it's a very blunt instrument and actually using it is likely to be suicidal.
I don't think we have any clear notion of what, exactly, the Warden can (and cannot) do with the prisoners.
Maybe they'r a blunt instrument. Maybe they can be more like an épée, or a scalpel.
Maybe it depends on the specific prisoner, with a variety the Warden can call upon depending on the need.
Also: the Well itself, and the Ley-Line nexus, represents a huge magical asset in any conflict. Harry needs to build up his own skill & power enough to handle that power, but in time he can doubtless channel magic that's many of times stronger than any spell cast from his own internal resources.
...
If Harry threatened to release something big league to force the WC to heel, it might work...once. But now the whole Council, including any who still sympathized with him at all are gonna want him deader than dead, ASAP. Even if he's safe from them on Demonreach (which isn't totally certain but I wouldn't rule it out), he's now more or less trapped there. Plus there are going to be other supernatural players no happier with him.
With the stakes that high, the outside world could try things like taking his loved ones hostage or the like, too. Even a lot of the 'good guys' might resort to such means if the stakes were that high. Plus, if he's corrupt enough to start doing stuff like that, he might get a Knight of the Cross after him. I doubt if even Alfred could be fully sure of protecting Harry against a Knight on the clock.
All in all, it would be a really stupid thing for him to try ...
You present this as Harry-the-Dark-Lord.
What if Harry is just fulfilling Bianca's headstone: He
diedpimpslapped the WC Trying to Do the Right Thing.
The WC isn't a monolithic power-bloc; it presents itself that way to outsiders, and tends to act en mass if it acts at all; but it's rife with internal rivalries and divisions. You can be damned sure the White Council is working as hard as they can to (subtly) surveil him, and a lot of the surveillance will be done by sneakier Wardens, many of whom have already held Harry in high esteem... and what they will discover is that Harry continues to
Stick Up For The Little Guy, continues to
Protect Mortals From Monsters, &c. In other words, act ethically and responsibly. They will remember -- what's the
very first thing Harry Dresden did when he became the WK? Destroyed the Red Court (who had been slowly winning the war, up until then).
So if -- for example -- Dresden comes to the WC with overwhelming force and says, "time for necromancy, folks... yeah it's dark and dangerous, but it's the only way we can take down the rest of the Blampires, so you WILL use it, this once..." he might just play to a more-sympathetic audience than Dark-Lord-Harry would.
...
Demonreach is a prison. It holds the Trouble, but somebody has to put the Trouble in there. That Somebody, usually, is the Wardens and the Council ...
They
can't.
Without the bond to Alfred/Demonreach, they cannot interact with the prison (except as interlopers).
Only
THE Warden can put a prisoner into a cell on that island.
Undoubtedly, the WC can assist. They can help ID & track-down beings who need to be put there. They can help whittle them down, make them vulnerable to the bindings. They can identify weaknesses, unearth True Names, etc etc etc.