Okay. So the rules for the map is unclear. The best solution most be to agree with the other player (I think most players don't alow characters to move through the walls with a move action). The second best solution is roll a dice.

Has Wizkids officially ruled that character that "has" a multibase dial doesn't count as a possession?

Yes. You don't ignore a character's base. Like was said above, things that make a character a character (having a base, having combat values, etc) is not something that is used or possessed: it just is.

After all, if you could ignore aspects of what makes a character a character, then whenever you try to PW my guys, I'd just say "You ignore my combat values. Now you have no idea what number you need to roll. Nyah-Nayh!"

(if it helps, you can totally imagine me sticking my tongue out and doing other silly taunting behaviors when you read that last part)

That depends on what the definition of "is" is.

Not quite. But Spiral could get around that... or considering the relatively short distance, you could just RUN around it. Last time I played on this, I had Iron Man RS 6 squares and I was able to shoot my "neighbor" (when I also accidentally forgot about first-turn immunity)... it's really not all that far.

With that being said, it would be up to the judge whether they would allow IT:Blocking (which was only created several years after the map was made) to shoot through it or to go with the intent of the map (that you can't shoot through or move through the force-fields). If there's no judge, roll a die and let luck decide.
If I was judging the event, my emotions may play more to it (whether it's a devastating difference that will make an easy win for one player) but I think I would go more with the intent... no matter how good Iron Man's targeting systems are or how powerful Cyclops' eye blasts, they won't get through Dr. Doom's force fields.

There's no way to shoot through the force fields. Not with IT: [blocking], not with Cosmic Spider-Man or any other character whose line of fire is never blocked, not with any effect that requires you to draw a line of fire.

The rest of the orange text for the map, which isn't in the starting post:

When counting squares to determine the range or area of any effect, you can’t count beyond a force field. (To determine if you are counting beyond a force field, draw a line from the center of the square where the effect originates to the square you are at tempting to count; if the line goes through a force field or through a diagonal bordered on the grid by a force field, the square can’t be counted.)

Thanks, Vanisher! I've only seen the map once and had no idea there was more to it. LOL! Rep to you!