Unless something has changed in recent code, 2X scoring applies to all CIU scoring including individual shot jackpots, the 'collect' jackpot, and the completion bonus.

I don't think you get two snake add-a-balls on the coffin MB stack. I might be wrong on that but I'm pretty sure the add-a-ball can only trigger once, even if you start another MB. You can relight the snake for 2x scoring/lock but I don't think you get another ball or ball saver (if you had 4 in play already).

If it is possible I would love to see it done. Will try it next time I play MET, but I distinctly remember trying this when the snake lock/add a ball was put in the code and it did not seem to work.

AAB exists in all 4 main multiballs. If you already have 4 balls in play (ie - coffin) it will simply give ball saver only.

Yeah the only way to EOL is thru all four CIU modes being played, then lighting CIU again for a 5th time.

I don't know why the option is even there for the 5+ million; one or two jackpots in CIU beats the score choice every time, and it doesn't help progression-wise...

After you complete the mode, victory lap scoring like this is plausible (with 2x). Until that point, you cannot farm one shot over and over for much.

Not to my knowledge

I might not be following your question but maybe this helps you:
When you are in CIU: you light the scoop by hitting Sparky, Snake, Coffin, Cross. This will light it to give you a change to collect (and exit) or keep playing. If you choose to keep playing, the scoop will (should?) stay available and you won't have to qualify it. I think the actual blue flasher might stop annoying you (?) but the scoop is definitely supposed to still be available.

..... unless you change an adjustment in the code!
On the 'hard' CIU scoop setting, continuing playing and not collecting/exiting will unqualify the scoop and you have to qualify it again. In the latest code this may have even become a default for all I know - its an additional punishment for risking your jackpot

The scoring in FWTBT is actually more in depth than just completing each shot (although the completion goal is unchanged).
The 3 inserts on the 5 shots work like a 'grid' - each shot will light the 'lowest' insert on that shot for like 1-2 mil. If you complete a "row" or "column" (ie - one shot to each = 5 in a row; 3 shots to one = 3 in a column) you are awarded more points like a super jackpot, this is usually 4-5 million. This isn't helpful to know in terms of completing the mode, but in a tournament setting a couple of easy double jackpots could be sitting there without you knowing it
Once a shot is complete its ~500k per shot until the mode is completed. FWTBT is not the easiest to complete, but it's probably the easiest and safest CIU to pick up some points. (although FTB is still better imo on a premium/LE thanks to the spinners)

Once all inserts are filled you've collected all these double jackpots and you get into victory laps which I think are common between all CIU modes. I believe they are combo based where the first shot is worth 1-2 million, and a corresponding spider arrow combo shot will then flash for a multiplied jackpot. I can't recall if this extinguishes arrows or not, but I do think the combos can continue. The shots all reset after a combo times out.

2X applies on top of all that theorycraft above.

That sounds like a possible glitch/bug to me. I don't see anything in that description that should cause that.