Welcome to the BA forum Madey, I hope you have an excellent trip to Austria and that you enjoy the experience.

I'm sure others will pitch in with their views but:
- I would not arrive that early: there is often a mini peak at that time for those travelling on the early services at 6:30pm, the Iberia services and those checking in rather too early for later services. I would leave it until 5pm if you can, just to avoid the hold ups when the Premium Security opens up, you can leave it until 7pm and still get a meal in the CCR. The CCR is good but I wouldn't stretch it out for that long.
- Personally I think 10 minutes in the WC is a bit anti-social but I'm male! Perhaps using the CCR facilities more (which is nicer anyway)? But it happens all the time.......
- You don't need to tip the CCR staff, unless you've had truly excellent service and you really want to show appreciation. Very few passengers do it, it certainly isn't expected, but if you think it is merited, then why not? However a few words of genuine gratitude will go just as far. This is only at the NY end, you certainly shouldn't tip in the Cathay lounge in T3!
- Above all, tell the BA crew as you board it's the first time you've been in First and you are looking forward to the experience.