Ret drops into trolls

I don't usually consider trolls as a drop I need to tech for super hard, but...out threat them, out shoot them, bring an appreciable amount of armor cracking and/or a way of scalpeling out the krirlstone, and you should be fine. Ossyan, Issyria, Elara, all work fine.

Vyros1+2 can usually put any troll beast into the dirt.

The melee attacks of warrior models in the Shadows theme all ignore tough, so that's pretty great. Elara2 and Vyros1 can both work well in that theme.

I play Elara a lot, and it really depends on the list you're running/what you're playing against.

I tend to build my Elara lists fast, fast enough that I can often get the alpha without feating. If that works out, I'll feat the turn after the alpha to put my second line into their backline, or to put Imperatus onto their warcaster with Ghostly + Sidesteps. The list often doesn't hit hard enough to grind out a long attrition game against an attrition-focused list, but you can usually get your opponent to make a mistake in trying to deal with the waves of dudes. 

If the enemy list is fast enough that I can't get the alpha without the feat, or if I can get enough high-value targets to put me up on attrition, or if they've got that Irusk2 spell that makes their CTRL area rough terrain, then you can consider feating early - if you managed to get a MK and the WW, then you probably made the right choice. However, I find that the threat of the feat is as important as the actually distance it gives you, and you don't want to blow it too early, because from that point on, your threat is predictable. I've gone whole games where I never feated, because the massive threat ranges put the opponent into the headspace that they needed to accept an alpha, and it just wasn't necessary to pop feat at any point in the game.

What was your list? I heartily approve of your decision to run out of theme, by the way.

I find the feat has three uses, listed in order of importance:

1) Threat range extension
2) Free charges for myrmidons
3) Ghostly movement tricks, like moving through obstacles and avoid free strikes

If I will get any sort of use out of it at all I use Elara2's feat for my initial charge. However, I often don't need it on the initial charge so I save it to get some free charges for my myrmidons later on. It's a very generic feat, one that will seldom be decisive but will always be useful. Don't hesitate to use it up front, as it's unlikely that you'll ever find a magic situation where it'll be decisive. I would only save it if you have a clever plan (like moving your troops through an obstacle to surprise the enemy) or if you want to keep it for a late game assassination run. A turn 4 feat just on Imperatus can be nasty.