1) Yes, though recreational wicking base layers will likely suit you appropriately for that duration. Merino = <3
2) there are too many variables for a simple answer. Body fat, adaptation (or lack thereof) of brown fat, diet that day, whether your base layer has wicked you appropriately, etc all play a role. Literally, air space = loft = warmth. There isn’t a magical material that gives you expedition puffer style warmth the thickness of a t shirt.
In your conditions, I’d wear a FE Halo over some 200g merino, others likely an FE Arctic. Some of the Weezle offerings are budget priced but have fallen out of favor.