I wanted to build a deck featuring Denethor and sons, and I specifically wanted it to be tri-sphere, because of all the different cards I could tap into, rather than an ally swarm.
Most of the time, you probably won't have that much to pay for with Denethor himself, but he can always give a resource to either son (let's face it, he;s going to give it Boromir, even if Faramir can make better use of it). Most of the time, Boromir will be doing the heavy lifting, but Faramir has his uses and they are many: card draw, healing, getting round condition attachments, are all nice toolbox things he adds to the deck, whilst Boromir is off killing things.
The main problem this deck seems to have is with questing, so not that strong in true solo. I'd be interested to know how other people would improve it (without changing the Hero line-up and turning it into a completely different deck)
If you want to do better questing, you need to increase the number of allies you have, probably especially Gondor allies with 1 wp, and increase the copies of Visionary Leadership you have in the deck. You can make room by trimming down some of the attachments and defensive allies. You've got a lot of deck space devoted to fine defensive allies and attachments for them, but any of your heroes is a strong defender with Gondorian Shield and with Boromir emergency defense can be covered. In a different deck I might want to setup Faramir with Wingfoot and A Burning Brand, but LeDenethor can't do much besides defend. If there were no Boromir I'd want to set up Faramir as an attacker with Ranger Spikes, and that wouldn't be bad here at all, but given the hero lineup he's going to need to quest. CelebrÃan's Stone should go on him.
In Service of the Steward serves no purpose in this deck, since the heroes are all already Gondor, and all it could do for one ally is increase his wp one, if Visionary Leadership is out.
One Gondor ally you should definitely add is Envoy of Pelargir. It costs two but immediately returns one resource to any of your heroes (since they're all noble/Gondor), and it's 1 wp Gondor. You want Mablung in this deck, he's 2-cost and 2-wp and Gondor. I think you should also consider Ingold Herald of Anórien Veteran of Osgiliath and Pelargir Ship Captain if you want to stay with the Gondor theme. Move Miner of the Iron Hills to the sideboard, you don't need them unless conditions are out.
If you're not attached to Gondor other than the hero lineup, I would drop Visionary Leadership entirely and ignore the allies I just mentioned (except for Envoy of Pelargir), and include 3x Entmoot, 3x Treebeard ally, and go for and ents. Thanks to Denethor you can play Treebeard on turn one, and Denethor can continue to feed resources to his sons to pay for ents on subsequent turns. Wellinghall Preserver can fix your questing problems.