Termidor SC can be used in sandy soil. You would still do the trench and treat the sand 6" x 6" around the foundation as termites will still go through that area to get to your home if looking for food, if this is your target pest.
If the footer is right below the dirt level, within 6 inches, and it comes out 12 inches, then you should make your trench out where the footer ends. In other words, instead of digging a 6 inch trench directly against the structure, you will move the trench away from the house where you can dig down past the footer. You should still treat the soil in between the trench and the house (where the soil sits on top of the footer) by just drenching the soil if you can.
Termidor SC bonds to the soil and does not move once it is applied. Therefore, it can be applied where a well or garden is present as long as it is more than 10 feet away from the well or garden or tree. We would suggest not applying Termidor SC along the side of the house that is within 10 feet of the trees. You should keep a close eye on this area of the house for termites, or you can of course always install some Termite Bait Stations in that area for monitoring activity.
You can either drill into the deck to get to the soil underneath, or you may need to remove the plank or two closest to the structure so be able to treat the soil.