If it were true open source I think the project would get a lot more contributions. The enterprise edition, with the "best" features, is very much not free.
This is absolutely true. They actually have a hostile attitude towards contributions. I used to work at a Magento shop (bad times all round) and my (extremely talented) colleague would often submit patches for long-standing bugs through the ticket tracker which would never appear in general release. Tickets would go for months without responses. It's commercial software at the end of the day. With an obscene price tag on it to scam cash out of non-technical business founders.
None as far as I can tell. There's OpenMage, which accepts contributions. But the problem is that Magento Enterprise is shit, and the Community Edition is shit with the functional bits removed.
Magento is very much open-core in all practical terms.
I didn't like my magento job much :/