https://www.rsracing.com/tech-wheel.html this link has ALL the information regarding how and where a tire ends up when it is connected to the lugs. Basically, back spacing is how far the edge of the RIM (not the tire) is away from the mounting flange.
By guessing
6.25"
5.25"
Back spacing is a wheel term. It is the back spacing of the wheel, to allow a larger tire to fit in the wheel well. It is built into the rim, so you have to order them with the proper backspacing, or officially called negative offset. A tire and wheel shop will have a tool that they can bolt onto your car, to determine maximun allowable offset, when ordering wheels.
Depending on size and steel or alloy it is approximately 5.5 inches
Yes they are both 6x5.5 with 4.5 in backspacing or 6x139.7
26"
Stock Backspacing is usually 4.5 inches.
265/75/16 on stock wheel without rubbing. Any aftermarket wheel, especially considering backspacing or larger tire size will likely hit the wheel-well liner.
Backspacing on the Teardrop 15" rims should be 3.75", backspacing on the 16" rims should be 4"
4.52" backspacing -12mm offset 16X7 6 on 139mm bolt pattern
The wheel will fit. You didn't specify tire size though.