Hi Michael, not sure if this is a DM or a post, however I'll try to break my answer to your Q2 into 4 parts.
My source instructions are (1) Natasha's trySwift email on how to prep; (2) Robert P's Wednesday presentation at AltConf with the warning about not to forget toolchains; (3) the other Robert's Thursday workshop at AltConf on participating in the trySwift community; and (4) the Swift.org GitHub readme.md
With preparation assistance of the two Roberts and hands on guidance by Mike on the Friday I got through the day (but even then had build/validation tests that evening).
So once I was home I decided to rebuild the system.
I have partitioned my 256GB MacBook Pro into two 126GB "disks" and over the years each WWDC I flipflop partitions so I have one current system and one beta system.
The requirement to have 70GB free was a major issue and hindered my setup for the workshop and the actual workshop day. Even my 1st rebuild i.e. 2nd build had issues resulting builds & associated tests running out of disk storage.
The storage was being eroded in the background by my iDrive; and unticking "manage" was not enough, and then attempting to delete the archive the OS had created also was not enough.
I worked out I have to turn off the iCloud Drive immediately the OS becomes active. So I then went to the first beta drop of Catalina; and without iCloud being active, and without vanilla Xcode installed I had enough space to have the freedom to experiment.
III. Latest Xcode beta and Swift clone installation
Having installed Catalina and the Xcode beta I believed I was in a good place; and SwiftUI worked with its canvas.
I then started to reinstall the Swift source. That is when I got the furthest with builds & tests but encountered the first error (top of the post).
Whilst working on that issue, the next Catalina beta and the Xcode tool beta dropped simultaneously.
From that point the SwiftUI canvas stopped, and despite your assistance I continue to struggle with the scripts.
Hence my decision to do my third clean install; I have some confidence that by meticulously following the instructions (I. above) on a naked Catalina beta I will come (close) to the end.
I believe the main issue is not the instructions per se, but me :) or rather my lack of experience with *nix systems. Despite 50! years of working with IT I only two projects (and a beta) where I had to learn (and then forget) AIX to achieve successful installations, PoCs, and benchmarks.
So what I believe I am missing, and what the instructions presume, is a conceptual model of how macOS hangs together -- I am gaining that by resolving each issue however I have yet to find the appropriate "dummies" guide to becoming a systems programmer of Macs!
The other presumption the instructions have is a detailed familiarity with *nix commands: that is more easily addressed by the usual "dummies" guides.
I think it would be unreasonable to expect the instructions in I. above to be more detailed.
Hopefully it is not unreasonable to expect me to adapt and become a more active part of the Swift technical community.
Thanks again for your assistance (and the aforementioned Roberts and Mike), I shall give you an update at the end of my day.
Ed - running on AEST :)