Hello. Finally it seems to happen to Slack and Rythmbox too. So I don't really know if is only an Opera issue or a system one. I'll try to reinstall MATE or the notifications library or whatever is involved in this thing.
I think the only long term solution to this problem is to actually compile from the sources and use the options that include Widevine and distribute that to Linux. I doubt Opera has any prioritised interests in this since it is a closed source browser
they are giving out no compensation.
I do not know if it is related, but firefox, having better html5 support than opera, plays the youtube videos in h264 while opera in webm.
If this support regards audio files as well, the problem becomes aac (which is supported by firefox and not by opera) vs ogg.
As for what you can do about it, read the post on my signature.
You are kinda lucky.
A friend of mine had an account at bet365 so I asked him to use my opera, log in there and try playing a live stream.
As it seems, all its live streams are in h264, thus they do not play in stock opera, and there is no fallback option for flash, thus having flash installed or not makes no difference. So all I have said at the other thread about opera's poor html5 support apply for your sites and distro as well.
One thing at a time...
For the installation part, you could just download opera's deb file, install it like usual by double clicking it and let dpkg do all that work for you Keep that in mind for next time you will need to install it fresh.
The missing o-menu button is a known bug that only affects unity users as it seems. Does it open when you press alt though?
If you really want to install an older version of opera, find its deb from here (go to "version number"/linux/ subfolder) and install it by hand.
Then put opera on hold so that apt won't upgrade it to a newer version like so
sudo apt-mark hold opera-stable