If she is going to be that fussy and complain about everything (sounds to me like complaining just for the sake of it and to be dramatic!), then I'd say you need to approach it with a different mindset. Her behaviour, actually, is nothing to do with you - it's HER issue, in which case I'd say don't waste energy on stressing yourself out when it sounds to me like she'll find something to complain about whatever you do. Some people are just like that with food - not happy unless they're complaining and having dramatics.
Keep it simple. Do some nice tea/coffee with a lovely but simple selection of biscuits and cake. It's her CHOICE whether to be pleasant and enjoy it, or whether to be dramatic and rude. It reflects on HER, not you. And for the record, I agree with you; I was raised to be gracious and thankful to a host, to eat and show gratitude for what is served, and if you really don't like something, then simply leave it aside or don't select it (if it's a choice and not already served on your plate). I wouldn't dream of turning up at someone's for afternoon tea (or dinner etc) and sit there complaining about what someone had put a lot of thought and care into preparing.
At the end of the day, no one is going to starve. They're going for dinner after - so a few hours after you, she'll be complaining to someone else about their food! Treat it as a win-win - either she will participate and eat something (win), or she won't, in which case you have some yummy biscuits and cake to enjoy yourself (win)!
I know it's hard, but try not to take it personally. She has the choice of whether to enjoy your lovely afternoon tea or not. Equally, you have the choice whether to let her behaviour get to you or not. Take back your power and decide that if she behaves badly, you will not allow her negativity to spoil your day.
Out of interest - what were you planning to serve? (What is she missing out on by being silly?!). I bet it would be delicious!