Yeah it it is always an interesting question with self employed. I think in 415 compensation definitions it is almost certain that the pay would qualify as 415 compensation. Even though it is "fixed on 12/31" I don't see any reasonable way of classifying it as anything other than pay earned while in the employ of the employer.
A section 125 plan can be set up to provide for payment of dental insurance premiums, but an FSA cannot be used to pay or reimburse the premiums and "reimburse a participant up to $400" sounds more like an FSA to me.