Your doctor should have told you what limitations apply. I don't recall as it has been a while, but I have had several tests over the years and can say the only dietary restrictions I recall was during the hour or so I was released to have a light lunch before a final scan. That is to say I able to have a light lunch during the testing period.
I don't think so, you can't be certain the decaf is 100% caffeine free. Any trace of caffeine will act to neutralize the test.
Emphatically NO.Period. NO.