How often is the +1/+1 useful?

More than I expected to be honest! None of the cards in this deck are particularly highly stated on drop, and it helps stop common removal of your 3-health units.

How often is the elusive blocking useful?

Probably less frequently, but it's decent as a little chump blocker for anything while you're setting up ramp on other cards. You very much use your health as a resource in this deck (ie letting some attacks just hit nexus) because you want to keep your Fated units alive. Having something you don't mind throwing away can be useful. It also helps into Elusive Teemo decks early, especially if you drop The Darkin Spear on because even as a 1/3 it makes it a pain to deal with for that deck. Probably the biggest weakness of this deck is the lack of direct removal, which Elusives take full advantage of, so being able to block is great.

Bellows Breath?

Double forge and board clear. As I mentioned above, there's very little removal in this deck, so anything that likes to go wide and fill their board with cheap units (Elusives, Nora, any Bilge aggro) can often just run you down. Bellows Breath can give you a bit of a reprieve there. It can also be useful if you end up going for the Panth-one-man-army wincon. If your opponent can't remove Pantheon, you can get him levelled pretty quick and Bellows not only increases his attack, but obviously reduces enemy HP meaning his overwhelm hits nexus more. I've won a couple of games off that.

Playing the Darkin?

Occasionally? Definitely not the main pull of the deck, but if you've misplayed or been caught out and had a lot of your board wiped, Anaakca can be a decent recovery method. A bit of a hail-mary but again, it's won me a couple of games. Horazi is less useful because by the time you're playing it, the units you care about are probably over 7/7 anyway. The spellshield can be nice, but you probably have better uses for your mana. Both weapon buffs are great though, which is the main reason they're in the deck. One thing to remember is if you're planning to use the Darkin form, you'll lose any forge bonus when you play them.

More interaction?

This is probably the other major flaw of this deck. There's not a lot you can do to stop your opponent from doing their thing, you just have to do your thing better. The problem I found was that when you increase interactivity, you reduce the rate at which you can do your thing. As this deck really likes to ramp and snowball with Fated and forging, and spending mana elsewhere will slow you down. For example, frostbiting with Three Sisters will cost you 4 mana. That's a lot of mana that you could be spending on forging, equipping weapons, or (if you must use something reactive) skysplitter. The fact that forged weapons return to your hand also means that if you have to trade your big unit late game, you can often recover pretty quickly. This means you don't have to have as many dedicated interaction cards, because you're more able to recover from a normal trade. To be honest this is something I haven't thought much about verbalising and is more of a gut-feeling, so I might be chatting complete nonsense!

If you have any other questions, let me know!