I've been considering finishing a few projects without any commercial mindset... just finish the games, release the source code under a permissive license and the assets as well. The project would be a living portfolio for me. I thought this can be a good way to join the industry. However:
Should i make every code from scratch? Since it's a portfolio, i'm considering using just the barebones for my engine: SDL + OpenGL + Bullet. Nothing more. Does this make sense or am i being just paranoid?
Should i make every art from scratch? I'm sufficiently good with environments: rocks, trees, terrain, etc. But weapons, characters and other art is really difficult for me. So i was considering using some CC0 art or collaborating with an artist for that kind. But wouldn't that defeat the purpose of the portfolio?
I don't really know, exactly. Technical artist mixed with tool/gameplay programmer? Trying to show my work as jack of all trades, actually. I just want to get into the industry in a good position. I've been a modder for such a long time that i learned some basic skills in diverse areas. Today i'm a technical leader in boring enterprise programming. Java, ERP's, DDD programming, wrist-cutting...
Jack-of-all-trades is not a job title (at least, not an entry-level one). This is not to say that having a broad range of skills outside one particular discipline is bad (it's good), but you still are going to need to decide which actual career path to focus on. From what you've described of your current career, "programmer" is probably the option you want.
This is a tricky one. From my experience (AAA) game studios don't want "jack of all trade" employees. My advice would to be think about you really want and where your talent lies, then shine on that specific topic, make it jump of the page (could be gameplay programming, tech programming, ai, game design, production, audio fx, music, shaders, art etc)?
For the other things you need in your portfolio it's perfectly fine to "borrow" other stuff, as long as it's legal