Well there's your problem. C7+ saw a lot of fixes and features, giving us a very usable video engine. However, C7 - C8.5 users will need to own a license to Quick Time Pro (~29.99). This isn't really published by Steinberg, any place that I'm aware, but if you own QT Pro, a lot of the issues you may be experiencing will be resolved, namely importing a video and not being able to see it.
Cubase 9-10 uses Steinberg's proprietary video engine which still has some weird flaws. I'm still using Cubase 8 Pro which has been stable for me for a few years now. I have the newer version of Cubase but haven't planned up migrating over to them yet until certain bugs are fixed.
If you are constantly busy, writing to picture and needing to make deadlines, I'd just encourage you to check out the software, Video Slave. You would need to another computer to host it but it would manage your video playback and work in sync with your DAW. You can still spot everything within your DAW and set Timecode, etc. The video will just be hosted elsewhere and you won't have to deal with the bugs and sluggish performance when working with the video inside your DAW. But, this setup seems only practical for working professionals given the cost to own a license and build a computer designed to handle it. It's definitely a luxury.
I’m on C6 and video works ok. You need to have the right codec (H264 seems to work ok on my Mac system) and match the frame rate of the video in the Project window, either by typing in a number that you get from the video or having it default to the settings of the video-it’s a checkbox. “Mux’d” MP4s wont allow you to separate audio from video, btw.
QT 7 Pro has some really helpful features for making a copy of the video and placing your exported audio into a video file, thereby replacing whatever was originally there. This is helpful if there’s location sound or VO on the original video and you want to replace if with a mix of that stuff and your music.