Question about targeting a Hexproof creature
The guide for judges handling rules infractions is called the Infraction Procedure Guide. The situation in question looks like a "Game Play Error — Game Rule Violation". The remedy section for that item says that a judge should first consider a simple backup. The backup rules say


  To perform a backup, each individual action since the point of the error is reversed, starting with the most recent ones and working backwards. Every action must be reversed; no parts of the sequence should be omitted or reordered.


The GRV remedy section then further says that partial remedies can then be applied to only a specific set of situations, none of which match the situation in question. It is not inconceivable that a judge could interpret these instructions as allowing them to do a backup to after you announced the spell and before you chose targets, but that would be a stretch, and essentially inconsistent with rule 721.1:


  If a player takes an illegal action or starts to take an action but can’t legally complete it, the entire action is reversed and any payments already made are canceled. [...] If the action was casting a spell, the spell returns to the zone it came from. [...]


The fact that the judge took their cue from the opponent when determining the remedy makes this additionally questionable. From that description it seems likely that they simply assumed that the opponent was correct, or knew what they were talking about.

In general, if you disagree with a judge's ruling or remedy, you can appeal in accordance with section 2.9 of the Magic Tournament Rules:


  If a player disagrees with a judge’s ruling, they may appeal the ruling to the Head Judge. In larger, Premier-level tournaments (such as Grand Prix and Magic Tabletop Mythic Championships), with prior approval, the Head Judge may designate additional Appeals Judges who are also empowered to hear appeals. They will be wearing the same uniform as the Head Judge.