The pavillion will give you a certain amount of projection if it's a bandstand but the mic and amp might be the best way to go, this might sound a bit mad but if you want to try it out drag the relatives outside and test what the range is then you'll be spot on when you have to perform, I kind of agree with your objections to option 2 as you use your thumb you could have a go with a pick it might not be that big a mountain to climb, worth bringing the number two options into your practice routine if you can spare the time but it does depend on how much time you have to prepare for the first session, if you've got a style you're comfortable and confident with then you should stick with it any change to the status quo will put a bit more pressure on you.
Have fun with that Johnnydoxx sounds like you'll have a great time
You don't stop laughing when you grow old; you grow old when you stop laughing.