Could you solve this problem by using a different external SSD for each coin?
Yes but you'd still need to remember to run it with the datadir command each time.
If you have an internal ssd big enough, you can make desktop shortcuts for it and set their images to the logo of the corosponding wallet and give them a good name so you won't get them mixed up anywhere.
Even if you put the datadir somewhere once, you need to keep reminding it as it looks for the config file in the same place and unless you try extensively fiddling with file permissions and links then it's not going to work just changing the datadir once.
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I thought that Bitcoin Unlimited used a separate folder by default, and it was Bitcoin ABC that used the same "Bitcoin" folder (as in "%appdata%/Bitcoin" if you are a Windows user).
I suffered this myself then I installed Bitcoin ABC in a computer that already had Bitcoin Core installed. I found out the hard way as when Bitcoin ABC overwrited files and destroyed my blockchain files. Luckily I didn't put my wallet.dat on there.
These fork developers are real jackasses, they should inform with big red letters that if an existing client is installed, it will overwrite existing files as it will install in the same path. A lot of newbies have got screwed by this or even worse, sending BTC to BCH addresses as the format is exactly the same.
I wouldn't even trust VM isolation. Pick up a cheap laptop to install altcoin software if you can afford it.
Nope, it does it on a lot of them.
Bitcoin Unlimited was the first to catch my attention by it. I'm not sure if any before them do it. (I've had litecoin core installed but I no longer use default data directories due to this issue - that's much safer of an option). Instead I use a different path for everything and then if I don't get a message box saying that there is an error and it goes back to resyncing from the start, I know I've loaded it wrong.
I was told the issue with BU had been fixed, but I'm not trusting that myself and doubt it was even true.
You're right about the VM thing, however, dual booting the computer with another operating system just for the altcoins might be a good idea if you can't afford another computer. I'd dual boot from an external hard drive though so you don't accidently make a mistake and put something on the wrong one.
