Is it possible to transition from an E-3 to a green card directly?

I had a person at the US consulate remark on the fact that I was applying for a third E-3 visa with the comment "you can't keep doing this indefinitely", I didn't challenge him, but this by understanding was quite the opposite, that there was no limit on E-3 visas issues; can you provide any insight into this?

If I obtain a green card by marriage, but then split up before the 2 year deadline, does that have negative repercussions on your ability to get employment-related visas? I've already been dating my girlfriend for 2 years and we've been living together for most of that, so it's something that comes up as a reason to get married, but I'm not sure if it's a good idea.