Cleric (-1 Religion) do skill check for Religion instead of Wizard (+5 R.) cause his Cleric or bug?

Each character is assigned a dialogue role depending on their personality traits and possibly background/class, regardless of their skill proficiencies. So there is no consistent way of making any one character the party's "face" or "smart guy".

It is a bit unfortunate that we're not allowed to use highest value, like most would try to in tabletop.

If it becomes too bothersome, you can turn on forced dialogue roll success in the game options.