An answer

9. What is the formula?
   This is not the world's most elegant solution, but it works:

Lead at 1.34 g/cm³
Tin at 7.31 g/cm³
Antimony at 6.697 g/cm³
Silver at 10.49 g/cm³

(11.34*(1-x) + 7.31(1+x))/2 = 10.49
11.34-11.34x + 7.31 +7.31x = 20.98
18.65 -4.03x = 20.98
-4.03x = 2.33
x = -0.63385742

Therefore:
.7890819 cm³ Lead plus .2109181 cm³ Tin weighs the same by volume as 1 cm³ Silver

Using the same formula:
.81692871 cm³ Lead plus .18307129 cm³ Antimony also weighs the same by volume as 1 cm³ Silver.

Add the two formulas together and divide by two, the final formula is
0.803005305 cm³ Lead
0.105459050 cm³ Tin
0.091535645 cm³ Antimony
= by weight by volume of 1.0 cm³ Silver

The proportions are remarkably similar to the proportions of Lead, Tin, and Antimony found in Lead type, forged by Gutenberg.