Backspacing and wheel safety

[KielTb]

So, let me start off with this,

Currently have an 06 ext 4.2 with 3.73s and the stock 17 inch rims.

I've read threads of people using ext rotors and calipers and fitting 16s with 4" combined backspacing.

My question is how do you know what is safe for backspacing?

Can the mounting surface of a wheel be literally anywhere between the two faces? For example our 17s sit on the outside face at I believe +55mm backspacing? ( correct me if I'm wrong)

Would it be safe with basically the opposite offset? Like get a wheel with the Mounting surface to the inside face of the tire so it doesn't rub against the calipers and inside the rims?

If I wanted to put 16s on my ext to have more options would it be possible to have just marks adapter and then find a wheel with enough backspacing?
Also any opinions of reputable wheel sales? Always been wary of buying aftermarket wheels and having them not be rated for the weight or use.
And last but not least opinions on wheel style/design?
I think some bike spoke style universal tuner wheels would buckle under heavy weight or a hard hit before a steel or aluminium mag rim like 2500/3500 8 lugs. How does shape size and design of the hub surface, spokes and lip effect everyone's rim choice opinions?