I've got this whole story fixed guys!
Well not as much as I would have liked it... The LBS and Cannondale talked to each others, Cannondale called the store that sold me the bike, etc. ONLY with the pictures the LBS took and sent to Cannondale.
Ended up that Cannondale told the LBS that there was nothing wrong with the dent, the structure is solid and it won't be a danger so I can now ride it.
I'm still not that confident though... I don't know, I wish someone could have done a frame test or anything. But well, I will ride it and if it breaks I guess Cannondale will have to respond of it in front of a judge eh. (Kidding, unless I get my whole face broken)
Thank you guys for all the answers on this thread!