Yes, the roll bar mounting location is horizontal.
Yes, the GoPro mount may vibrate a little, but the real risk is that the vibration cracks the mount, and will eventually send your GoPro flying off the car and into the weeds, where you might find it later. Some of us used a safety wire to secure it to the mount.
Yes, the best solution is another brand stronger mount system. I have not tried the one you mentioned, but I have used the following with excellent reliability and ease of use.
Ram-Mount (You can buy direct, or buy from a reseller for slightly less cost)
Use this part to attach to the roll bar with a wide tip flat head screwdriver.
http://www.rammount.com/CatalogResul...5/Default.aspx
Use this part to clamp to the mount.
http://www.rammount.com/CatalogResul...5/Default.aspx
Use this part to attach your GoPro camera housing.
http://www.rammount.com/NewProducts/...5/Default.aspx
RAM also sells an
all-in-one kit for the GoPro, but the roll bar mount part of it is different, and I can't vouch for its strength. I can say the one I listed above should work great, and are the same parts Helix used for their cameras.
Hope this helps!
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Roll bar mount
Clamp
GoPro mount
The whole thing should look like this, but with the different base I posted above. The steel U strap in this photo takes too long to mount. Be sure to use the flexible metal strap.
