Emerge Unscathed and
Shelter are both "nuts & bolts" picks I'd happily make, regardless of if my strategy is aggro or control. I actually cut
Emerge Unscathed because it was too high-impact in my list for a while (I was running 2 of them). A cube needs a certain number of these "nuts & bolts" picks that aren't the most exciting picks to function, so don't shy away from protection tricks if they interest you - your drafters will learn to love them, and if they don't, that says more about design incompatibilities leading to creatures being over-prioritized than anything bad about the spells themselves. Your proposed scenario of a "top-decked
Shelter" rotting in-hand seems a bit absurd to me; yes, that can happen, but that can also happen with your inflexible aggro-combo cards; at least
Shelter cantrips and can do lots of work in more control-oriented decks! I think it's a far more versatile tool than you're giving it credit for, personally.
Addressing your protection question more generally, of the non-
Mother of Runes /
Selfless Spirit options, I'm personally pretty interested in
Hedron-Field Purists, but it's obviously not addressing your protection-seeking efforts as far as aggro-combo goes here, so I'll skip over it.
Eight-and-a-Half-Tails has all the problems you point out (mana-intensive, color-intensive), so I'd pass unless your environment is very friendly to it; it's a super-defensive mana-sink that I don't think most aggressive decks would want.
Benevolent Bodyguard is really low-powered as well, significantly below
Emerge Unscathed by a county mile.
Void Grafter has the unfortunate problem of being
, which isn't usually an "aggro-combo" card, so you're not really addressing the problem you're posing in this post. I think perhaps you've lost the forest for the trees; none of these proposals really help aggro-combo, which is the deck you're looking to help, and aside from
Hedron-Field Purists, I'm not sure what deck really wants them more than
Shelter or
Emerge Unscathed.
Personally, I'm a big fan of
Mother of Runes,
Selfless Spirit,
Shelter, and
Flickering Ward. Those four pieces together provide very flexible tools that aggro-combo, prowess, control, and even "midrange" decks love having access to, and will compete over.
Flickering Ward is a seldom-played gem that has an incredible amount of versatility to it; being able to bounce it back to your hand and reallocate and re-calibrate your protection effect is extremely cool, and it's been a top pick around here since I added it and we saw it in action. If you
really think you need the most power possible,
Emerge Unscathed is typically stronger than
Shelter, but it's less-desirable to non-aggressive decks, and by the looks of things, aggressive decks are getting plenty of tools in your cube already.