The bad news
You can't run the GFCI outlet off of one half of the branch circuit as you propose, as it's illegal to put a 15 or 20A receptacle on a 30A circuit. (The receptacle itself isn't protected properly against overload in that case.) You'll need to scrap the receptacle, or put it on a different circuit/run.
The other problem you have is providing a disconnecting means for the pump -- right now, the only disconnecting means for it is the branch circuit's breaker in the panel, and in order for this to be Code, the pump location needs to be in sight of the panel as per 430.102(A) and (B). Otherwise, you'll need to toss a disconnecting means for the motor and controller into your box instead of that GFCI you wanted to stick in the 3rd gang, and clearly label both switches as to their function.
The good news
In turn:
- Article 404 won't stop you from connecting switches in parallel like that at any voltage, provided everything's running off the same branch circuit and the switches are being used within their ratings.
- Leave the panel bonding in its current state (i.e. bonded for a main panel, unbonded for a subpanel).
- You can put a SPDT (3-way) switch on the 24V side as Harper describes provided you have a spare low-voltage wire to bring 24V down to the pump location.
- There is no limit on splicing as long as the individual splices are made in accordance with 110.14 and the box fill is not exceeded
- Stranded THHN is not a problem as long as the terminations are rated for stranded wire