I would use the same recommended pressure as your stock 185/55R15 tires. Unfortunately as this is an inch up you would need to find the right balance and sometimes it could be tricky. The range of being ok is extremely huge. So as long your tire is not off 5 psi + or - you are safe.

When you dont feell anything at all then balance is right

When you rotate, did you not balance it?

Steering vibration could mean a lot of other things but since you rotate it just before might be worth to check it out.

When rotate it is better to at least do balancing for the front tires first, everytime i do rotate i will ask to check or do balancing for the front the cost is not much some only RM5~10/pcs.

Not knowing the Load Index of both of your stock and replacement tyres, I think you should be fine and safe.

2pcs is fine. front or back depend ur familiarity with car

If your wheel setup is not staggered, put the 2 new tyres at the rear, & bring the old rear tyres to the front.

asking = dunno? just follow majority say behind lol.
i put front becos i tried both liao. maybe u should try both side as well

What tyre size u talking about?

UC7 should be quieter than VP4 but performance wise VP4 is better. VP4 is same category as CPC7 and Primacy 4.