I don't really understand what happened to your device, I frequently boot between OS's and variants of the same OS (stretch/buster), never any unexpected problems. I can point you to some info that might let you figure it out if it ever occurs again, and also maybe short circuit some of the faff of a full reinstall.
The four different key combinations just launch a different kernel from a different boot partition [boot, recover, boot2, boot3], technically you can flash (with dd in linux or the flash tool) any kernel for any os to any of these, the recovery one doesn't turn on some hardware features so its best not to re-use for a proper os.
If android is not booting then you can use the flash tool with the tick boxes to only re-flash the android parts, it is known that mounting the android partitions rw from other OS's is enough to cause android to stick its middle finger up at you* so if you want to nose about you need to just mount ro.
The original tri (and/deb/sfos) boot made use of stowaways where debian and sailfish used the same partition with debian on the root and sailfish in .stowaway/sailfish folder that was then mounted as the root point by the modified sailfish ramdisk. From the sounds of it the newer sailfish is moving to LVM. I've had no reports of anyone trying the new tri boot so I don't know what it does just now but LVM does sound like it might be a nice solution as it would avoid the OS's interfering with each others files but still allow say 50/50 split until the user decides which is more important and then zero flash station (zero data loss) resize/removal of the lesser/unused one once the more used one runs out of space. Basically its more likely that deb/sfos would mess each other up than that they'd mess up android. I also still like the stowaways form as it lets you completely avoid any need to allocate space between os's.
* - I don't recall what form that took, it could have been a reboot, not something I ever tried myself.
