Unless you are recording a replay instead of a demo, you can't really shorten the footage without using external programs.
There are, however, many quality-of-life commands that can be used to navigate demos way better, such as:
(in console)
demoui - (or just pressing shift+F2) will open a really nice demo navigation UI thingy, which can be used to fast forward, pause and jump to specific 'ticks' of the demo, which is essentially the time elapsed of the recording. If you're making a report you can fast-forward through it until you find evidence of the rule being broken, then state the respective tick in the report itself so anyone watching can type this tick into the 'GoTo' box and be taken directly to the right part of the demo. The UI also allows you to change camera perspective with the 'drive' button. The controls for this are shit though.
sv_cheats 1 - Does nothing to help the demo on its own, but will let you activate other really useful commands such as
thirdperson - puts you in third person duh
and
glow_outline_effect_enable 1 - essentially gives you wallhacks in the demo. You can see all players through the map and it makes it really easy to spot a hacker, for instance (they lock onto people through walls).
EDIT -
@Zeprus :
hidepanel all - applies to spectators and removes these brown borders at the top and bottom of the screen, giving you the full fov. Also particularly useful when watching suspected hackers, especially snipers.
That's all I know. I like demos. I'm like a demo man.
Wait...