It's nice that you were conscious about what kind of hardware you used and, what kind of torque was used on that hardware. I think you over thought some of it though, that's just my opinion. I installed the base plate on our 2011 Honda CRV and it too had very little room for the safety cables that were supposed to fit here and there. Well, I trust my own work and, torqued everything to specs etc and didn't use the safety cables.
I just installed a base plate (actually two separate ones) on our new toad, a 2011 GMC Sierra 1500 Extended Cab 4x4 and, the Blue Ox safety cables did not fit where it showed in the pics. And, based on the style of install, there's no way on this earth that those two, independent base plates will ever come loose so, I did not install them (the safety cables) on this unit either.
And yes, Grade 8 bolts are harder and stronger than grade 5, they will have worked for you just fine. In that particular situation, you'd have never broke those grade 8 bolts. But, it's up to you, it's your vehicle. Good luck.
2004 ITASCA HORIZON 36GD, 2011 GMC Sierra 1500 4x4 Toad '08 GL 1800 Gold Wing
Retired-29.5 yrs, SDFD, Ham - KI6OND
Me, Karla and the Sophie character, (mini Schnauzer)