I have a script, that checks pulseaudio chans, and if sound is played, the script postpones xscreensaver. So, watching movies is never interrupted by screen lock, while idle working state and auto-locking works as usual.
About mouse cursor: just move it to the right bottom when movie started, it will be invisible. No problem to me.
Very much possible. One should not go though the pain of dualboot to watch videos.
The only real problem I see is performance. Qubes OS renders video in software mode, solely by CPU.
So, you should have modern CPU, give qube with the player multiple vCPUs for better performance.
And use the proper player, like smplayer (mpv-based or mplayer-based). Because some players, like totem or something that comes with gnome is playing video much slower and worse in software mode (lags with movies that mpv plays well). So, stick to smplayer, mpv, or mplayer. Can be installed from Fedora fusion.
I have modern Intel CPU that plays 4k without frame drops, but it works on the edge for it. If bitrate of the 4k file is higher that usual, it can drop frames. Blu-ray may be also demanding, I do not know for certain.
Also useful thread about video capabilities of Qubes OS, mostly regarding youtube: