Need Advice for Plant/Storm

Spirit Tree is optional -- kind of like a stationary Regen Aura. I find its best use is during longer, tougher fights, or where your team plans to "make a stand" by bring foes to a particular location instead of running to each group. For example, in the ITF, you can use the Tree by the Bridge when a teammate goes to pull the generals, by the computer console and when beating on Rommie. It is a lower priority power, but nice especially for melee characters with Resistance to help them heal up the damage they take.

I love Hurricane on my Stormies. I consider it a "highly recommended" power rather than a "must have," and it takes some practice to learn how to use it well. It can be both an offensive and defensive tool. Turn on Hurricane and hide around a corner, and foes find it hard to get through the doorway to come after you -- and if they get past, their ToHit is severely debuffed, so they can't hit you. You can pick them off easily. You can push stragglers into AoE powers or towards the tank/melee fighters. You can lock foes into a cul de sac, debuffed and prime for the getting smacked around by Tornado, Lightning Storm and your pet. And it makes a good "panic button" power for when you and other squishies are getting overwhelmed in melee -- but you are correct that it doesn't help with ranged damage. (I will sometimes look for ranged defense on my stormies for this reason.)

Used well, Hurricane is very "situational." I leave it off most of the time, then only turn it on for short times in ideal situations. I have it bound to a button on my mouse, so I can pop it on with a flick of a finger. Using Hurricane at the wrong time can actually hurt a team, so it takes some practice and experience. "Highly Recommended" though.

I generally take the Ice APP on my Stormies. You have no self heal or endurance recovery, and Hibernate can cover both of those. Also, the two AoE damage powers require some positioning, and Storm is the best at positioning. Your Roots Immob will hold foes in place for the entire duration of Ice Storm. Seeds of Confusion is a cone, and so it Frost Breath -- You can have a regular attack chain of Seeds, Roots, Frost Breath, Roots. And the Armor is defense-based, and the single target blast is decent.

One of Plant's biggest weaknesses is a lack of single target damage, so the Earth set is worth considering. You get two single-target attacks, an short-range AoE, a heal and a defense-based shield (that is really ugly). I generally skip Hurl, but Seismic Smash is just too good.

Steamy's defense is minimal -- its main benefits are resistance to Energy, Fire and Cold, and stealth. Most of the damage in the ITF is lethal, so the resistance won't help much. But the Stealth does. I generally take Super Speed for travel with my Storm Controllers, as Steamy + SS = full invisibility. A stealth IO will do the same thing, but I like the battlefield maneuverability of Super Speed to get in postion for casting cone powers, moving around while running Hurricane or getting in position for Lightning Storm, or even in position for using Gale.

Stealth provides no benefit over Steamy Mist, and doesn't provide a benefit to teammates. Grant Invis provides some benefit, but not enough better than Steamy to me. Full Invis takes two powers to get, and is inferior to Steamy + Super Speed.

Well, the problem here isn't specific to steamy mists. It's due to the -def effect in the Cimerorans attacks. Let a chop or two get through and suddenly you've been minced. Unless you have a sizable amount of lethal/smashing defense they'll quickly cut through it. (I have a fire/ff/stone softcapped v. smashing/lethal that has fallen to some unlucky hits and collapsing defense on the ITF.)