If I say attack, does that mean Transporters can't lose the -2 atk while doing a transporter attack?

That would be correct-- they would retain their printed attack values.

My question: Would choosing the speed value allow a HSS/RS/Charge character to move at full speed to make an attack? Or are those power "immune" on account of a technicality ("halve its speed value for the action")?

The new speed value is a replacement value, not a modifier. Hence, charge/RS still get the movement cut in half.

Page 13 of the current rulebook clearly states that halving the value is a replacement, so CB has no effect.