If you are playing an event where it is announced that PDGA rules apply, then the Offical Rules of Disc Golf apply, whether the event is sanctioned by the PDGA or not. The Competition Manual only applies to PDGA events. If no announcement has been made regarding the rules, you can play by whatever rules your group or the event participants agree on, including the PDGA rules.
The meaning of “first” in the rule is the common understanding of when the disc first enters a state where it is in violation of a rule. One common pair of rules that can be violated during a single throw are OB and Mandatory. As soon as a disc enters the restricted spaceplane it is considered to have missed the mandatory, whereas a disc is not considered OB until it comes to rest. Therefore, the missed mandatory happened first.
No. To make calls during tournament play, you must have been authorized by the Director as a Tournament Official. Passing the test does not make you a Tournament Official (referred to throughout the rules as an Official). Additionally, Officials have restrictions on making calls depending on whether they are playing or not. An Official (including the TD) who is playing cannot act as the sole Official for calls that affect players in their division. A non-playing Official can be the sole person to make a call where rules indicate an Official may make the call. A spotter can make calls (for example, regarding the position of a disc that has gone out-of-bounds) if they are also an Official. If they are not, their call should be considered as input for a group decision.
All PDGA-sanctioned tournaments have a Tournament Director. For non-sanctioned events or casual play, if anyone has authority over the players, they can take on the responsibilities of the Director. If no one wants to be the Director, then you will have to play without some of the functions of the Director. For example, there may not be any appeals of group rulings. Some Director functions may be available in other ways. For example, the course signage should tell you in what order to play the holes, where any out-of-bounds is, and other things that are normally covered in the players’ meeting or caddie book.
Benefit of the doubt only comes into play as a tiebreaker when the group cannot make a decision, for example if two players see the disc as safe and two see it as OB. If a majority of your group thinks it’s OB, then it’s OB.