Hi @heathrobinson,
Your primary question. In my experience there’s never been upgrades across major versions like 5.1 to 5.2. Manual upgrade only. This isn’t synaptic’s decision - it’s the repositories synaptic is using or method of installation that determines this.
There’s a few ways you could be seeing multiple versions in synaptic.
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Did you by chance add Oracle’s repository per THIS PAGE?
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Did you download a *.deb file and install from that?
As for trouble booting, try disabling 3D under Display. That’s been causing fits for a few people - like me.
Safe? I abandoned 5.2 and reverted to 5.1 due to horrible performance. I’m running VBox on Ubuntu-MATE 16.04 LTS. You may want to mention the version(s) you’re talking about.