Shifting bridge and nut
Instruments- discuss specific features, luthiers, instrument problems & questions
That's a very interesting point about the tuners. I hadn't thought of that.
Out of interest, I detailed my problem to the customer service department of the supplier and this was their (somewhat dubious) response:
Thank you for your very detailed inquiry.
I talked to the repair shop at Roosebeck and they are more in the position that their instruments were not designed to be "customized", and that using the instrument with any fewer strings than designed will cause an imbalance that is causing the movement.
They say the only way to keep the bridge and nut from moving is to glue it down. They use Loctite glue.
Of the various options you have compiled, it seems if you want to keep the two pieces from slipping or moving around, but don't want to glue it down, you may want to try spraying the back of the pieces with non-permanent spray adhesive, they stuff they use on fabrics, headliners, etc. If you search on Amazon, you may find a liquid non permanent adhesive. This would be the preferred option versus permanent glue or screws into the wood.
If the adhesive solution does not hold tight enough and you have to resort to screwing things in place, go peruse the Home Depot isle where they stock screws and brackets, you can most likely find a very small screw/bracket/washer that you can screw into the sides of the bridge to lock it in place.
Should be a selection of washers like this at Home Depot for you to find the right size, then you would need a very small wood screw.
Sorry we can't offer an exact solution, but it looks like you have explored all the options.