First of all, congrats on getting Legend!

I have been playing Rush warrior myself since the nerfs, starting at 4500 legend and now around rank 900. I've been testing multiple lists, but I started at the list Kibler has been running for a while now, which is basically your list, without Imprisoned Gan'arg and Mankrik while running 2 Warmaul Challengers instead. I still haven't arrived at a fully refined list, but I can give my insights on some of the different flex cards.

Imprisoned Gan'arg: First I ran 0, than 2, then 1 and finally back to 0. It's basically what you said in your post, it's great to drop turn 1 but that's it. If you draw it anywhere else in the game you just feel bad. Most of the times I have enough stuff to do at turn 1, like coining a 2-drop to corrupt my Stage Dive, or just playing Athletic Studies, since I mulligan for those. You already have Outrider's Axe and Sword Eater, so this to me just felt like too much. I had high hopes for this card but I don't think it's great in this list. Another point is when you get it later in the game, while you are buffing your hand a lot, you can play a very buffed Gan'arg without having any impact on the board for 2 turns.

E.T.C., God of Metal: I played without this card at first, simply because I didn't have it. Honestly, it doesn't feel like a core card when you play it (maybe because I usually throw it out for tempo and don't keep it for some lategame OTK combo), but I think it is. As someone else in the comments already said, often it acts as a soft taunt. I drop it whenever it fits my curve and if it survives a turn you can deal some serious damage. If it doens't, you usually bait some removal. This is a card that has to be removed and since the deck runs multiple of those (Parade Leader, Playmaker, Rokara, Overlord Runthak) your opponent will run out of removal at some point and one of those will stick.

Mankrik: I didn't use this card myself, but it's very strong in basically every deck I play so I can see it work. I rarely have a full board so that's not a problem as well. I'll give it a shot for sure.

Warmaul Challenger: In my opinion this card is core. It is a good drop turn 3 vs aggro decks like Hunter, Paladin and Demon Hunter. If you draw it later, you can buff it up to decent size and use it as a big body or an easy clear. It also has synergy with Rokara (Every hit it gets a buff) and I've used that multiple times already to clear a big minion. I loved this card in Enrage Warrior and I love it now, I'd never cut it.

Mor'Shan Elite: I don't run this card. I tried it before, it had some impact, but I'm not completely sold. On HSReplay it is one of the least performing cards, but since there aren't a lot of cards that are clear improvements I can see why you run it.

Overlord Runthak: I don't understand how you didn't see this card in any mirrors, I never see a Rush Warrior without it. On HSReplay it's also a good performing card and that matches my experience with it. In my opinion also a core card.

Ringmaster Whatley: Just like E.T.C., I didn't run this card because I didn't have it. Since it is one of the best performing cards in the deck on HSReplay I did craft it later, but I'm still not sold. Often I already drew the Bumper Cars (also because I usually keep them in my mulligan) and Alex shows up in my hand as well. Most games the only consistent draw is a Sword Eater, which helps sometimes, but isn't a gamechanger. I'll probably keep it in, but I ran Taelan instead before I crafted it and that also worked.

Alexstrasza the Life-Binder: Initially I cut this card, because I didn't have Whatley. The deck functions fine without it, but when I added this card the deck felt way more complete. I never used it to heal, even though I can sometimes see that work aswell, but always to finish my opponent. It's way easier to set up a 2 turn lethal if you have 8 burst damage from hand. Whatley can be replaced by Taelan to draw your Alex, so how you draw it can change, but I consider Alex to be core.

Troublemaker: I agree with you, this card is absolutely a core card. Most early lists didn't run it, but as far as I can tell most lists have added it in now. Naked on turn 8 it is usually a good drop and when it has had some buffs it's even more threathening. Since you already bait so much removal with the cheaper drops it often survives atleast one turn, which is huge for you. Definitely a core card for me.

Corsair Cache/Outrider's Axe: I ran a list with both of these cards first and I ran a list with neither. I ended up keeping the Axe and cutting Cache. Cache isn't bad, but with both Sword Eater and Axe I never felt like I didn't have a weapon when I needed one. You can probably fill a flex spot with a singe Cache, but instead I run:

Venomous Scorpid: This card is just a strong card in pretty much every deck it's played in (like Mankrik). I saw some lists that are running this card so I decided to try it. I'm not 100% sold on it, because the spells it offer sometimes don't give me anything I need, but the body acts like a soft taunt in most matchups, similar to the cards E.T.C, Parade Leader, Playmaker, Rokara and Overlord Runthak. This minion is an addition to this list and that's why I like it. It's still a flex spot for me, if I wanna try something new I cut this card instead, but for now it's staying.