Is it possible to activate a switch for an ability that does not roll any ? Or... if I roll a single with Mendoza's Flamma Veritatis, can I use it to activate the switch instead of causing damage?
Yes, you can activate a switch even if the ability itself doesn't have any successes. Yes, you can spend the hits from Mendoza's attack to inflict burning instead of dealing damage. That's one of the reasons he's so good: even if your opponent rolls literally all shields, you can just spend any hits you rolled to burn them instead.
Wait... doesn't cancel in any case? Like when a model tries to disengage, for example? If you spend them for a trigger can't be cancelled?
Switches take place before succeses get cancelled by shields. Also, using succeses in switches uses them, so, for example, if you use all of them to burn your opponent with Mendoza, or if you use them to mark your opponent while trying to disengage with Eclypse, you won't have any left to do damage/move away
