Why is one side is a MDFC arbitrarily considered the “Front”? unlike with transform cards that doesn’t seem necessary. But I may just be missing something.

The rules have to have a designated front and back. It has to specifically be something in each zone. For example, if it’s in your library, and it’s a creature on front and a land on back, you can’t tutor for the land, but you can tutor for the creature.

I do think it's confusing that separate mechanics involving two-sided cards all have the sun/moon symbol in MID. These symbols seem intuitively tied to daybound/nightbound, but they don't seem have a good logical association with Disturb or the other transform cards. It's not intuitive that you could have two cards on the battlefield with the same day/night symbol and one transforms with day/night, the other doesn't. Was it looked at to use different symbols for Disturb and Transform?

All TDFCs in previous Innistrad sets (barring Eldritch Moon) used the sun and moon symbols, so I believe we just continued what we had done before. I agree, in retrospect, that daybound/nightbound should have made us reconsider that.

Assuming Transform is making a return in one or both Innistrad sets, is there any worry that it will cause confusion with all the MDFCs in Standard?

No. The back sides of transform cards don’t have mana costs.

I have some DFC questions. So what happens if I have a Blackbloom rogue in play and I cast MoonMist? If I have Oracle of Mul daya out Can I play Blackbloom Rogue from the top of my Library? Is there any Traditional DFC that can get to its back side without using Transform? Is there currently any MDFC that can get to its back side with Transform? How do MDFC get to there Backside?

You can only transform a card with transform. Moonmist is merely creating a different way to transform cards with transform. A MDFC doesn’t have transform, so it can’t be transformed.

Do the single triangle/two triangles on the top left of MDFCs signify anything specific? With transform cards we had sun/moon, moon/eldrazi and the planeswalker symbol for the flipwalkers that all fit with what the two sides meant. do the triangles have any meaning or is it just a way to say "side 1/side 2"?

It just means side 1, side 2.

Do the showcase frames mean that we are going to see a mechanic with a unique frame in every set going forward? While I could see XLN/RIX transform cards, Sagas, and WAR walkers being showcase cards, there's nothing from RNA, GRN, or either recent core set that fits the bill.

The showcase cards don’t require a mechanic with a new frame. That just happens to be the case for Throne of Eldraine.

Hey Mark, was there a reason the Serra's Sanctum didn't get a Transform card like Cradle did?

Yes, we didn’t make one.

If you found a time to reprint a flip card, such as in a Master's set, would you reprint it as a flip card, or alter it to be a transform card?

Flip cards would get reprinted as flip cards. Changing them to double-faced cards would change their functionality.

With regard to the morph/transform debate. Morph is a mechanic that is practically anti-linear. While you get a benefit from including multiple, in many cases the benefit you get is the ability to include this creature where you otherwise wouldn't. It increases the flexibility of your deckbuilding. Transform on the other hand ALWAYS has a flip condition, meaning you're forced to build around it more. While the condition may vary, its basically always something linear. EX: Path of Mettle

Morph is not anti-linear. The issue isn’t can you play one by itself but does the mechanic encourage playing other cards. Morph’s bluffing aspect does encourage playing more morph cards. The mechanic isn’t strongly linear, but it’s linear. Transform, in contrast, is a mechanic that says turn the card to its other side. That’s all it does. You can design transform cards to be linear but the mechanic itself does nothing to enable that. The flip condition can be straight-forward like mana or even so simple that it will naturally happen. It doesn’t inherently have to be a build-around. The reason the question was hard was because a lot of early transform cards were linear, not because the mechanic is inherently linear, but because the individual designs were. The question was testing your ability to look past execution to function. I hope that helps.

I'm new to MtG, so sorry if this is answered somewhere I don't know, but if a card allows a search for a land, can a person search for the flipped version of the legendary flip lands from Ixalan?

No. You can only search for the front of a transform card.