Looks like the Armageddon gretchin kit is quite limited. The 5-grot doomerposts and valrak's 5-grot rumors weren't far off—5.5 grots per sprue. There are two pairs of swappable top and bottom halves (red/purple and green/blue in my crappy edit), and one bottom half with two top options (yellow, white). There are no extra heads, the heads are all molded to a pushfit body part. The 10 pictured grots are the only swaps you can make without cutting.

HOWEVER, this is a hopium post. GW clearly knows how unpopular the monopose boyz were. I don't think they'd expect such a sparse gretchin kit to be a hot seller for years to come, especially when they're gonna want to raise the price per usual. They've gotta know most people aren't going to want to buy the exact same 10 (but kinda just 5 1/2) grots several times over. 

So I think these pushfit grots aren't meant to be our new gretchin kit. They're just an easy-to-build kit to stick in the starter boxes, that only cost them a half-sprue allocation to make. I think we'll get something more proppa when we get more new models in 11th. It might not be so heavy on the spiked helmets either, the 2e homages might be more of a specific thing for the Armageddon box.

The sparseness of this gretchin kit makes me think we're definitely getting a better (non-pushfit) gretchin kit later on