If a person plays Hilde and uses Hilde's E before siren's call E is that legal?
Siren's call question
Sure. The "you may not increase" restriction only applies after you play the Enhance on Siren's Call - any increases before playing that Enhance are legal and will not prevent you from using Siren's Call's own Enhance later.
Note that you cannot play Hilde's E before the "cannot increase" has happened, unless your opponent has canceled the ability, leaving the cost payed, but not the inhibition established.
The restriction doesn't apply until after you play the Enhance on Siren's Call. It doesn't say anything to the effect of "you may not play this ability if..." etc. All it does is put an additional restriction on what you can do after the Enhance is played. What you do beforehand makes no difference.
I more meant that there is no way to take advantage of the cost of the ability before the restriction kicks in, unless the ability is canceled.
A bit of extra information, but darklogos can be creative with the rules some times.
Funny thing it is not me most of the time. But glad to know the order.