Hello, and welcome to the forum!
I probably can't be much help with your main questions, since I've never dealt with UEFI, GPT or made a NetBSD live CD. A tiny part of getting started might be to read the man pages for boot(8), mbr(8), gpt(8) and some of the references in them.
When it comes to dealing with limited bandwidth, there's one idea that I've used worth throwing out there. If you have a portable WiFi access device (e.g., cell phone or tablet) or can borrow one, and can get to a decent public WiFi AP, then you can use that to download an install iso. That will have "everything" on it so it's a convenient download. Once you bring it home and copy it to a real computer, you can then download a (small) checksum file via a more trusted connection and verify the iso.
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