#1
I like em scale the most. But will playing in C or a help me better understand the tension created by intervals so that I can play what I hear in my head.
#2
Intervals are intervals. The same intervals are found in the Em scale as in the Am scale. (Same thing with the G major scale / C major scale). Intervals are just the distances between two notes.
#3
Not in the least. The tension created by intervals is just that: intervals. The tension comes from the way the notes relate to each other, not the notes themselves. The actual key is completely irrelevant.

I would recommend playing scales/keys other than Em, just because using other scales will add some variety to your playing, but it's not going to help you understand intervals any better.
Guitars
Schecter Hellraiser C-1FR, C-1 Classic, Hellraiser Hybrid Solo-II, Special Edition E-1FR-S
Orange Rockerverb 50 212
Basses
Yamaha RBX374 and Washburn MB-6
#4
The only way you can understand the tension between intervals is harmonically and using any note as a root. So use your Em scale you like, select E as your root and begin adding other notes to hear the tension. Then use another note, like F#, and use the notes from before against it. Continue doing that with all 7 notes being used as the root in turn, and you will then understand them. Make notes - mental and written - and you'll be fine. Enjoy and good luck
#5
To play what you hear, you need to play a lot by ear. And you need to hear ideas in your head. Learning to play in all keys doesn't really have to do with being able to play by ear - I mean, if you learn all scales, it doesn't mean you can play by ear, you just know the fingerings. But of course it helps. Without knowing where the different notes are located, it will be really hard to play by ear. Playing by ear is all about ear-fingers connection.

But yeah, just learn a lot of songs by ear.
Quote by AlanHB
Just remember that there are no boring scales, just boring players.

Gear

Bach Stradivarius 37G
Charvel So Cal
Fender Dimension Bass
Hartke HyDrive 210c
Ibanez BL70
Laney VC30
Tokai TB48
Yamaha FG720S-12
Yamaha P115