There's a used Starboard carbon paddle that I am considering. These costs more than I imagined as the only paddle I have ever used were cheap aluminium adjustable ones.

My concern is that it is too long for me and would have already been glued (No idea what glue). Would I be able to unglue it and shorten it? How big is the risk?

depends on what glue it has been glued with and how much you need to cut it down. If it has a permanent Glue and is way to long then just cut it of and get a new handle.

I bought one on gumtree for $25 very proud of myself , it lasted two months before it snapped in half luckily not at a crucial time . The real issue with second hand carbon paddles is their history ie knocks and bumps , mine looked fine on the surface but obviously not Oh well 2 months 25 bucks

Oh 1 more thing , I have snapped 2 new aluminium and the 1 second hand carbon paddle so bought a brand spanking new carbon sunshine paddle and couldn't be happier , very worth the money $350 .Next to it is the before mentioned second hand paddle broomstick down shat and sloppy fibre glass job that my wife has dubbed the widow maker 2015 because of the weight and density I now think of it this way , its the motor that keeps the craft going

Shaft

Kind of like buying a raffle ticket.....could be a little harmless fun as long as the ticket isn't too expensive
My last punt was a Carbonerro Bombora $350 but still holding together so far