Rules apply to units right? When your unit dies, they essentially fold into another unit, whichever unit is nearest. Your rules literally say "must join a friendly unit." That night one of you described viper as being a "one man line" which is not what "join" means. Join as in you are now a part of that unit, and therefore play by that unit's rules. Skirms are totally valid, as is arty guard. If you are line and join arty guard, you play by arty guard rules.
"When there is one skirmisher left, that skirmisher has to find a friendly unit. However, that skirmisher does not become line infantry, so that skirmisher could take his spacing." This is ridiculous... one guy just shooting on his own, lovely... I believe we call that Rambo.
I seriously think there is some new gen conspiracy going on here or something because I've never heard this before after 1500+ hours on this game. I've been here since 2012. I've been to a couple of getty's events and it never ran like that as I remember.
I don't know where ya'll came up with this hot mess but I suggest you change it. Literally, the point of being in a reg is to play a part of a unit, not as an individual. When a skirm joins a line, he isn't a skirm with line, he is LINE. When a line joined skirm, he isn't a line with skirm, he is SKIRM. When cav joins line, he is LINE. You play by the rules of the unit you are in, not the reg you came with.