I'm not that knowledgeable about NHL contracts, but I don't think the contract actually starts if he doesn't play in the NHL for more than nine games. At that point, it's just a way for the management to show it believes in the player. The only thing that would have an impact (save for a signature bonus, maybe), is like Preds33 said, a contract spot.
Would agree. Plus the salary is pro-rated for time on the NHL roster so it's not that relevant other than it counts towards the 50 contract limit.
The reason they signed by the 15th has likely nothing to do with development camp and probably more because of the transfer agreement between the SHL and the NHL to avoid a penalty fee (at least that used to be the reason in the past). Leaves them flexibility.
Probably the same situation with Arvidsson although I don't know if he still had a contract with SAIK but Fiala did sign one with HV71 for 2014/15 earlier this year.
In the post-draft interview on the Preds website DPoile said that they would leave Fiala the choice as to whether he wants to play in CHL or HV71 next year. Which points towards this contract out clause otherwise he simply wouldn't have the option.
HV71 season starts end of August with the Champions League, so we'll likely get an indication of his plans by then.