I forget exactly why aluminum is off-limits. I believe it has something to do with some kind of byproduct that's produced. When you're straining, you can strain into a different pot or container, but don't put it back in with the plant material. Put that off to the side, and then refill the pot with plant material with fresh water. You'll keep on repeating the process of boiling, filtering, and combining. Finally at the end, you'll have all your combined boils which you'll then heat one last time to reduce.
I'm really no caapi expert, but since this is last minute, I'll tell you what I can. You'd probably be fine with 50g, but maybe what you'll wanna do is brew it all and then you can drink half. If it looks like you should drink more later on, it's there for you, and if not you can refrigerate it.
The tbsp of lemon juice is at your discretion, but that should be fine as well. Good luck, let us know how it goes, or if you have any other questions or need clarification.
"Science without religion is lame. Religion without science is blind" - Albert Einstein
"The Mighty One appears, the horizon shines. Atum appears on the smell of his censing, the Sunshine- god has risen in the sky, the Mansion of the pyramidion is in joy and all its inmates are assembled, a voice calls out within the shrine, shouting reverberates around the Netherworld." - Egyptian Book of the Dead
"Man fears time, but time fears the Pyramids" - 9th century Arab proverb