Guessing we're talking about 2nd gen cars here ?
In an ideal world you'll want virtually zero, or very low offset on front and rear, which is hard to find.
I've fitted a couple of sets of BMW wheels, the first we're 8.5" x 17 wheels from an 8 series I think, offset from memory was 19mm ET which works out around 4.5 inch backspace. The rears needed an inch spacer to fill the arch properly.
Current wheels, needed to clear the front brakes, are 18" aftermarket staggered for 5 series, so 18" x 8.5" with 19mm offset (same width and backspace as the last set) on the front, and 18" x 10" with 13mm offset on the rear, these also need an inch spacer to get the right position, so total rear backspace is about 5.75 inches as you say.
Some folk have squeezed an 11" rim with a 315 tyre but on mine it still rubs both sides with a 295, but it is lowered a bit and I do throw it round the bends alot
I think on a 9inch rim could go 5.25 or 5.5 inch backspace.
Any reason you want 16" wheels ? The tyre choice and sizes is very limited compared to 17" or 18". I'd personally go for 17" staggered 8.5 and 9.5" set for a 5 series fitment if possible.. If you're using spacers use decent bolt on 'adaptor' hub centering type.
can't help on the seatbelt though I'm afraid..