This is such a great thread! It synthesizes so much of what I love about the TB community - a love of seeing the world and a sustainable lifestyle. Having spent some time in SE Asia at different seasons, you are dead on with looking for natural fabrics, and merino wool will never let you down. Years ago we had a great company in the U.S. called Ibex, and they made some great merino tanks and tees. They've unfortunately folded. Icebreaker has a lot of options, but they're very expensive, tend to be cut very form-fitting, and in my opinion, aren't made as well as they used to be. If you are near an REI, they are starting to offer their own merino line, and so far I'm impressed with it.
Whatever merino tops you wear, I recommend getting several single-use packets of Woolite for hand-washing your clothing during your trip. You'll actually have enough in one packet to share with your friend.
Additionally, in terms of merino underwear, I will recommend Icebreaker for that - at least their siren style. You can also put inserts in the siren bra to provide more modesty.
In terms of linen, a few years ago someone on here introduced me to the brand Elizabeth Suzann.
https://elizabethsuzann.com/ Her philosophy is sustainable, slow fashion. Her team doesn't start making the item until you order it; expect 2-3 weeks for it to arrive, if you live in the U.S. I have a pair of her Clyde Culottes in linen and I just about lived in them last summer (humid Eastern seaboard summer). They will relax a bit with wear, but they are well-made and worth it. I also have a couple of her Georgia tees and Georgia midi dresses and wear them frequently; I like to wear the midi as a tunic over leggings. I'm not much for dresses and skirts, but she has several styles of those in linen, silk, and cotton canvas.
Shoes: Like others have said, the Mephisto Helens are great. Comfortable, stylish, and totally worth the money. It will rain lots. Wet feet are horrible. Wet feet in wet shoes are even more horrible. So, even though you'll be wearing sandals, make sure to bring a pair of wool socks to warm up your toes once you get back to your hotel room, because the A/C will make you freeze. (I wear Smartwool toe socks with my sandals to stay warm whenever I wear sandals on a plane - which is usually only when I go to Asia.)
I have yet to try Luna sandals -
https://lunasandals.com/ - but I've been looking into those. Allegedly, they're also good for running, assuming you've already prepped your body for barefoot-esque running instead of in cushioned shoes.
Shawl - at least one. A good wool shawl should be relatively easy to find - LikeMary, on Amazon, sells some lovely and well-made ones. Be mindful that they are quite large and made of somewhat heavy weight wool, so they will not pack down like some others will. NxN makes a square merino handkerchief that is much smaller, but keeps you warm and packs well. I've used it as a scarf around my neck or over my shoulders, and as a small blanket for my daughter when she's cold during trips.
https://www.northxnorth.co/shop/meri...-handkerchiefs
I'm so excited that you and your friend are taking this journey together. Safe travels and much joy!