Flipping, copying, and transforming

[AmbivalentDuck]

I have 3x Reckless Waif and a Volrath's Shapeshifter in play.  I cast Mirroweave making all creatures copies of my Volrath's Shapeshifter.  I discard Bloodline Keeper, and pay to make a Reckless Waif transform (1. Merciless Predator is the side facing away from the table? 2.  Does it remain a copy of Shapeshifter? ).  Then, I sacrifice 2x Waif to something just to get rid of them.  Now I discard Rune-Tail...and I happen to have 30 life.  The Shapeshifter flips if I'm following this correctly. I cast Mirrorweave again targeting the card that was Reckless Waif (What card is the Shapeshifter now a copy of, Merciless Predator or Volrath's Shapeshifter?).  I discard Erayo.  (4. What happens?)

[Delha]

Most of this can be answered here. Interactions with copy cards are discussed explicitly.

I'll edit this post later to reply more thoroughly (assuming nobody else beats me to it), but I've gotta take off just now. For a start though, you've only got 4 creatures/vampires on the table, meaning you can't activate Bloodline Keeper's transform ability since you need 5. Did you perhaps mean 4x Reckless Waif instead of 3?

Edit 12/16: Pls disregard below and see newer post.
Edit: Looks like I've got a few more minutes, here's the short version:
1. Only transformable cards can actually transform. Transformable cards that become copies of other cards retain the characteristics of the card they copied, even if they do transform. That means if you Mirrorweave a Reckless Waif into a Bloodline Keeper then transform it, it flips onto the Merciless Predator side but stays a Bloodline Keeper (unless I've completely misunderstood the rule explanation).
2. Yes it remains a copy of Shapeshifter.
2.5 Both copies of Rune Tail die, since you have the original Shapeshifter and the Waif copying it.
3. Assuming you hadn't pitched a Rune Tail, it is a transformed Shapeshifter, copying a Bloodline Keeper.
4. Same as in 2.5

Note: I'm assuming you started with 4x Waif and sac down to Waif + Shapeshifter after step 2.

[Delha]

Going back, it looks like I kept missing parts of your scenario due to rushing through. Sorry about that. Here's a breakdown of the actions you're listing, as well as the results from each.

Initial Board state: 4x Reckless Waif, 1x Volrath's Shapeshifter, 30 life.

Action: Mirrorweave targeting Volrath's Shapeshifter.
Each Reckless Waif becomes a copy of Shapeshifter. You now have 5 Shapeshifters on the board, they are all 0/2 because Mirrorweave is the top card of your grave.

Action: Discard Bloodline Keeper
All Shapeshifters becomes copies of Bloodline Keeper.

Action: Waif A is a copy of Shapeshifter, currently copying a Bloodline Keeper. Activate it's transform ability.
Waif A physically transforms, meaning the Merciless Predator side is now facing away from the table. It is still a copy of Shapeshifter, and currently a Bloodline Keeper. It is NOT a Lord of Lineage.

Double-faced cards are considered to only have the characteristics on the face currently showing. In zones other than the battlefield, double faced cards always default to showing the front face. When a creature is affected by Mirrorweave, the text physically printed on it is overwritten by what it's copying. Initially, the Waif text is being replaced by Shapeshifter; after transforming, the Predator text is being replaced by Shapeshifter. From there, it's Shapeshifter ability copies the Bloodline Keeper in the grave.

Similarly, if you had used Mirrorweave to turn the Waif into a Stalking Vampire, activating the Transform ability would change which face of the Waif was showing, but it would not become a 2/2 Screeching Bat.

The back face (with the moon symbol) of a double-faced card is effectively blank for all purposes other than checking if the card can transform or not. Cards with a normal card back (brown w/ MtG logo) cannot transform. Activating the second ability of Waif A does effectively nothing, activating it for the original Shapeshifter will do literally nothing.

Action: Sacrifice all your creatures but Volrath's Shapeshifter and Reckless Waif A.
Both of your remaining creatures are now Reckless Waifs. Waif A is still showing it's back face (Predator side away from table), but that is overwritten by the Mirrorweave which makes it a Shapeshifter. From there, the top card of your grave is a Waif, so it copies those characteristics.

Action: Discard Rune-Tail.
Both Shapeshifters becomes a copy of Rune-Tail Ascendant, the flip triggers for each go on the stack (in an order of your choice). As a state based action, both die due to the legend rule. For sake of progressing through the remaining steps, I'm going to pretend this action didn't happen.

Action: Cast Mirrorweave targeting Reckless Waif A.
The original Shapeshifter becomes a Volrath's Shapeshifter. Since it already is, nothing changes. Waif A is visually Predator A now, but all the printed text is already replaced by the first Mirrorweave, which turned it into a Shapeshifter. Copy effects completely overwrite the prior characteristics of a card by default. Mirrorweave doesn't consider Waif A to be a Waif turned Predator turned Shapeshifter, it just sees a Shapeshifter, and copying that info back onto the original doesn't accomplish much. When Mirrorweave finishes resolving, both creatures become vanilla Shapeshifters since the top card of the grave is now a non-creature.

Action: Discard Erayo.
Both shapeshifters becomes copies of Erayo, Soratami Ascendant. Then both die to the legend rule.

Bonus: If you pass the turn intead of discarding Erayo, Mirrorweave wears off and the back face of Waif A that has been showing all this time finally becomes relevant. It is a Merciless Predator, at least until your opponent's upkeep. At that point, the spells you've cast this turn will cause it to transform back into a Reckless Waif.