Be that as it may, your problem with Shops is that aside from running 13 Spheres, they will also run Wasteland + Strip Mine. If it were only the Spheres, Shelldockul is definitely *the* plan.
I've seen earlier versions of the Shelldockul deck running Amulet of Vigor. I know it's janky outside of the combo, but at least it can be cast off Dark Ritual mana, and if you had two of them in play, you get to pay for the Shelldock Isle's U activation with itself since you get to untap twice. Unfortunately, it doesn't solve the Wasteland issue, and again, it's terrible on its own, and Time Vault + Amulet of Vigor is not so hot, when you could just run Voltaic Key and a better Vault deck instead, if that were the case.
Without adding more dead cards like Cloud/Amulet/Dream's Grip/Twiddle, passing a turn is *seriously* a problem, because Unlike Oath, you pass the turn with half your life and majority of your deck Exiled. Oath at least doesn't do anything drastic and disadvantageous to its own gamestate when you play it. Overall, you will discover that you would need three "dead" cards in your deck: Shelldock, Emrakul, and the untapper. Mirror Doomsday only used two dead cards: Channel and Mirror, and neither screwed you over if you held it while comboing out.
The main reason I like Channel + Mirror is that once Channel is running, Sphere effects become largely irrelevant. With one Sphere effect, the combo of Ancestral into Lotus into Channel will cost you 2U instead of U (The 1GG is of course paid for by the Black Lotus.). When looping, you just pay an extra 1 life with Channel to pay for the mana required by the Sphere. As more Sphere effects occur, it gets progressively worse, of course, but Channel + Mirror lets you sidestep a lot of stuff (Like Leyline of the Void.), and Channel-looping is largely aided by stacking draw effects then killing yourself (Ancestral on the stack, kill yourself. If your loop will end up with 10 cards and Ancestral is on the stack, you will get all 10 cards in this loop, then AR is in your yard.). Normally, Sensei's Divining Top is your best friend here. Stack two looks and a draw: One look before you draw, the draw, then one look after you draw.
In 200 goldfishes, I have yet to fizzle out with the Mirror Loop. The next cheapest combo is the Desire stack (UB), and while it has less dead cards (Only R/D or Beacon since Desire in a combo deck isn't too shabby.), the catch is you can't be holding R/D or Beacon in your hand, and that's a big problem. Jace is NOT an option in Doomsday right now. I've tested Jace in Doomsday enough times to know that this is one deck that won't be buddies with Jace. Maybe See Beyond has potential, though.
Personally, I don't think Shelldockul is a valid plan A for Vintage Doomsday. I actually play Shelldockul + Tendrils in Legacy, and I get away with having Emrakul as my plan A in Legacy, but that's not quite the story in Vintage, where you not only worry about Wasteland, but Strip Mine, too, so a single Pithing Needle doesn't solve your problems. This is why I believe that though Mirror and Channel are dead cards on their own, that should be the core kill in all your stacks, unless you're buried under Sphere effects or facing way too much countermagic. It's also not impossible for one to just cast Channel into Emrakul, which is like a pseudo Show and Tell for you, although it costs 15 life and GG, and my last Vintage Doomsday deck already featured one Bayou and one Trop, so it wasn't impossible to hardcast Channel, either. The only problem is, your dead cards become Channel, Mirror, Emrakul, Shelldock, and potentially, your Twiddle effect.
To be honest, I was fine running Doomsday until the M10 rule changes happened. R/D was a really awesome trick at the time with the Mirror loop (I was recastin Desire for 100.), but when R/D could fetch only SB cards, it became significantly less powerful. The fact that the deck wants both FOW and Street Wraith in place to really maximize your protection and free draws into the kill also proved too difficult as FOW and SW really compete for slots. Taking away FOW means you're running a heavy B-based deck, and I think GWSx is better at that game. Taking away SW means you will only have Ponder, BS, Top, possibly Gush, and AR to draw into the stack, as opposed to all of those +4 other cards. Of those options, only Top and Gush draw into the stack for free.
Anyways, that's my input for Doomsday at the moment. Until M10, my magic career's health was literally tied to Doomsday's viability in the meta, so I definitely have a lot of interest in reviving the deck if it's possible. At some point, a compromise has to be made, and I'm not sure which one to make of the seven...
1. Do I focus on a plan that dies to holding the combo pieces in hand? (Shelldockul/Mind's Desire)
2. Do I focus on a plan that dies to Wasteland? (Shelldockul)
3. Do I focus on a plan that dies to countermagic? (Mind's Desire/Mirror)
4. Do I focus on a plan that dies to Sphere Effects? (Mind's Desire/Mirror)
5. Do I focus on a plan that is, on average, the least risky kill of all, but costs the most mana? (Mind's Desire)
6. Do I focus on a plan that could potentially fizzle out on me? (Mirror)
7. Do I fill my deck with dead cards? (Focusing on more than one plan)
That's my cost-benefit analyses for each plan, and what compromises need to be made for each plan. Hopefully, from here, we can look into ways to mitigate these compromises and come up with the best hybrid.