You can add tags to your post to say spoiler when you're starting a thread. They're not predetermined, you just type in whatever you want them to say. I know there's a way to do it on replies, as I'm on another Invision forum, and have seen it done there quite often. I think I've done it a couple times as well, I just don't remember exactly how. @DC would have to answer on that one.
As for you only being able to post the link instead of the video being embedded, did you use the link insertion tool or did you just put the hyperlink in your post? If you used the link tool, then no it won't embed. If you just put it into your post the same as any other text and give it a second it will automatically embed. Also you said you were on your phone, so that may be another issue to address.