I'm kinda a henna virgin too, but I've done my research.
1) It's safe to henna over previously dyed hair.
2) I'm not sure of the color effect it would have on your hair. But then again, no one would be able to exactly predict. What you should do (and what I'm going to do) is test it. Collect some hair from your brush or comb and mix up some henna. Smear the henna in the hair you collected and see how you like it. I forgot how long to leave it in, but I'll give you a link at the end of this reply. I've heard that henna takes about 10 days to actually settle (not sure how to word it) so w/e collor you get right away won't be the end result.
2) I personally don't trust myself with henna for the first day xD For me, I might be able to ask my grandma since she's used henna before. But, if you want, you can go to your salon and ask them if they would be able to use henna. Just be sure to make it at home before the day you go to the salon.
Here's the link I go to for Henna info: Henna for Hair
Check out the free ebook, check out some youtube videos. Go to the techniques page on the link I gave you too. It shows you how to text your hair, how to mix, and the step by step on how to use henna.
From 1 henna virgin to another, hope this helps