I had incorrectly visualized from your description that the whip was mounted on top of the arch which would have made the arch a natural platform to form an rf ground for the whip. As it is no matter what you do you will lose energy into the arch. Yes, you certainly can place the tuner below decks although with a good installation
you should not fear mounting one above decks (they are usually designed to do this) unless you just don't like the sight of it being there.
The main point is that a good rf ground be provided to the groundling lug of the tuner. If below decks it is not, therefore, as critical (except for lightning
protection) to make the arch an antenna rf ground because it is not geometircally immediately below the drive point of the whip.
If you desire the arch to be a good rf ground and the arch has four "legs" then each leg should be directed as straight as possible via the #14 wire bundles to a thru-hull, if possible.
Copper is not as passive as is the alloy used in bronze thru-hull fittings. I don't know that the benefit of a slightly imporved resistance to marine
growth is worth the expense of having to pay for expensive thick pieces of copper which can still erode away without the benefit of good zincs. Keep in mind that Old Ironsides did not originally have to live in a hot marina with other vessels generating all kinds of electrical
effects with onboard electrical
systems, etc. In addition, they did not have the benefit of good bronze metalurgy and understand just how to make a good passive marine