The answer is no, 4k makes lenses issue more visible so a cheap lense will look worst in 4k. BUT Sony lenses looks great on the a6300 because there is in body lens compensation, the images are transformed inside, aberrations are removed, distortion is compensated and in the end it looks like the lense is optically perfect. If you want you can disable it in the menu and your lenses will look bad again :-)
It's a great feature it means you can get great image quality from cheaper made lenses. I also have the a6300 and the 18-105 and this combo works very, very well, I leave my canon lenses at home more and more.
Well it's more complicated that what Stephen said. First thing first, it depends on the law of the country you are in. In europe where I live in, the autor is the owner the product unless you sold the rights to someone else. It is impossible to totally remove you from a part of the rights and even if you sold the right you still get paid something everytime your video is air. So in this case you could ask to be paid for every client that uses the video you made. If they didn't mention your name it's a fraud too so you could get a compensation for that.
Check that with you lawyer, and if you don't have one, just let it go, it's a large law firm, you probably have more to loose in attacking them than them.
I have the canon 17-50 2.8 and I love it, great all around lense for aps-c, also knowing that you have a 3x digital zoom with no loss of quality with the 70d (by croping in the sensor) it gives you enough zoom range to be covered 90% of the time. I have a 70-200 that I almost never use thanks to that. The 24-105 is great with a full frame camera but I find it too narrow on an apsc sensor.
Useless. The footage are ugly. Nothing close to something that would deserve to be called "a Pretty Freaking Cool Video"
Internal recording is not that bad, you just have to use a mic that have internal preamp like the rode vmp. The choice of an external recorder depends on your budget and needs. If you want to go dirt cheap I would advise you to get a Zoom H1, then you can pair it ither with a rode smartlav+ (lav mic) or with a shotgun mic like the zoom videomic pro or the takstart. You can also use the zoom to feed your camera so you don't have to sync and if you do have to you can do it easily.
Also there are the sony a6300 & a6500 that are worth looking at.