So I activate Multiattack.
Take free action to attack 1 of the 2 characters with an assimilation token, they make their shape change roll, so I make a ranged attack against a figure with no token and KO it.
2nd free action, I target the only other character with an assimilation token, because I still can't target the first one. He makes his shape change too. At this point, rather than letting me continue the action and making a ranged combat action against a character with no assimilation tokens, the Multiattack immediately just ends? So, I got to activate Multiattack and take 1 free action within the multiattack, and even got to begin the second free action of the multis track, but because it became apparent that the action would not meet the conditions necessary to allow the action, it just ends? I got to activate Multiattack but because I activated it illegally, the game just says "well, at least you tried, why not just stop there and we'll call it close enough to being a legal action"? How is that the same thing as making it a legal action?

Wrong, sir! Wrong! Under section 37B of the contract signed by him, it states quite clearly that all offers shall become null and void if - and you can read it for yourself in this photostatic copy: #I, the undersigned, shall forfeit all rights, privileges, and licenses herein and herein contained, et cetera, et cetera... Fax mentis incendium gloria cultum, et cetera, et cetera... Memo bis punitor delicatum! It's all there, black and white, clear as crystal! You stole fizzy lifting drinks! You bumped into the ceiling which now has to be washed and sterilized, so you get nothing! You lose! Good day, sir!

For the same reason with RCE. I have one character in range and another outside range. One is a Viable target so I activate RCE and they roll Shape Change. They succeed, the game does not now allow me to make a ranged attack on the character outside my range. Instead the action ends because there is no viable targets.

Shape change specifically says it can cause an attack to end if there is no valid target. In this case, a target still exists, they just have no assimilation tokens. Can't I just make the attack hoping I will KO a piece and then my opponent will use his free action from Big Figure to Mind Control my cube and use it to attack a figure on my team with an assimilation token? Letting me finish that Multiattack would actually increase the likelihood of the action again becoming legal. You have already begun a Multiattack that is likely to end in no attack being made against an assimilation target, why does it matter whether or not you do the second half and simply increase the likelihood of you ending the action legally, regardless of how unlikely it is?

If you cant use the second action of Multiattack to actually attack, can you use it to move?

How would it become "Legal" even if you successfully hit, you did not target a character with an assimilation token.

If you hit a character with your first action and give it an assimilation token, then your second action "MUST" still target a character with an assimilation token since you have not fulfilled that requirement yet.

The only way the second action would not be viable is instances of evasion. Which at that point you already declared the action and could not change it to another.

Well, not quite. During the first attack, your opponent may use New Mutants or Young Justice or Justice League International to redirect damage and kill off the only figure with an assimilation AND put the figure that just got hit into stealth, leaving you with no assimilation targets in range and line of fire. In that event, could you take a move action with the second action?

Okay, let me try and break this down again.

My opponent is running Big figure, "The Mind Controller", Target of my Assimilations 1, Target of my Assimilations 2, The Martyr 1, and The Martyr 2 (a borg himself). I have Borg cube and a Target of my opponent's Assimilations on my force.

Assimilated 1, Assimilated 2, Martyr 1, and Martyr 2 are all in range and line of fire.

My borg cube activates multiattack and targets Assimilated 1, but he makes Shape Change. Instead, I attack and kill Martyr 1. Big Figure triggers and moves up Mind Controller, who is in stealth.

Action #2 of Multiattack: I target Assimilated 2, but he shape changes too. Why should the multiattack immediately end? Why can't I attack Martyr 2 with the hopes of KOing it, hoping my opponent will use Big Figure and hoping he will use Mind Controller to mind control my Cube and succeed, hoping he will use my cube to attack my Assimilated figure to meet the condition of the multiattack? If my opponent doesn't do all of that for me, the multiattack would have ended in me not attacking an assimilated. That would make the action illegal. But, taking that really crazy risk at the end brings back a small sliver of hope that the multiattack will end with an attack being made against an assimilated. If that happens, the action will be legal because the multiattack will have included an attack against an assimilated.

Does that make sense?

Sure, but only if that action that is providing movement includes an attack that targets a character with an assimilation token.

What if I give them a power action to running shot a character with an assimilation token that is currently too far away? What if I don't count it out correctly and end the running shot out of range and line of fire of all assimilated figures?

Then you preformed an illegal action and your opponent should call a judge over "immediately".

How is Big Figure giving a Mind Control Action?

THIS PLACE IS GONNA EXPLODE!: During your opponent's turn, immediately after a character friendly to Big Figure is defeated, Big Figure or a character friendly to Big Figure can make a close combat attack or a ranged combat attack as a free action.

Does your judge have to sit at your table all the time?

Generally, yes. I ask the most questions and start a lot of threads on ridiculous rules questions. I'm sure you have noticed them. I have also caused the creation of other realmser's threads, like a few of RavenProject's rules questions (he is one of my judges, and he knows my games are loaded with questions that would leave even Harpua questioning the answer for a moment). Unfortunately, I usually forget most of the barrage of questions by the time I get home from events and am not able to post a thread to get the answer. I do when I can though.

I am not 100% positive, but I was fairly sure there was one random figure with a special power or trait that let them use Mind Control as an attack instead of action. Maybe I'm wrong? But I thought a figure like that existed.

What if the second action is an action to charge (granted from an Infinity Gauntlet) a KC/Hypertime figure and they make their KC roll? Should I call the judge on myself for completing an action that I didn't know was illegal until after I began it?

The idea of an ability that can be activated but then checks for whether or not it could be activated only after it resolves seems like a really problematic ability.

What if there is a target with an assimilation token that is out of range at the beginning of the Multiattack, but which can be reached with Running Shot. Is it legal to activate Multiattack, attack a target without an assimilation token and then RS to attack another target that has assimilation tokens?