I can't comment much on what happens when they get snagged... I have always been able to pop my baits and get unhooked by backing up.... 1) pops baits 2) free-wheel (almost) tension on ball and rigger line 3) raise other ball 4) back straight back and lower then retrieve ball... worked every time so far
Now, as far as attaching to the boat.... Start out with a Rube Goldberg 2X8 cut to fit on top of the gunwales and four C clmps... put the bases about 8 or so inches inboard on a second 10" or so piece of 2X8 stacked (glued and screwed) on the long 2X8 You now have a "baseline" setup (cheap) to work from as to ow it will fish in your boat... Start out with one rod on each down-rigger and see if you would want the bases higher... farther back, inboard or outboard...
Mine are mounted on the splashwell barricade at gunwale height and I am forced to fight/land all fish on one or the other side. (Not a Targa, 19' Magna with e fully barricaded splashwell running between the two gunwales and deep like a Lund Baron)
My riggers are manual canon 60" arms with "mortar tube" rod holders (2) for each rigger ( I don't like the ones mounted on the riggers...when the SHTF they are a pain to work around) And I have two extra holders so I can move a rod out of my way (mounted on the gunwales about 15-18" toward the bow on each side).
With your Targa I would think height would be the biggest issue (you may want them higher than the gunwales).
I have used D' riggers for everything from Salmon/Trout to Walleyes on Mississpppi/Illinois/Missori River and inland lakes on big water on the Great Lakes. Getting them mounted where they are as convenient to work and comfortable to work is a key component of their success.
Any thing else I cn help with just ask...