Multiplication Rounding

Firstly, the interpretation of a value on 64 bits is only a mental perspective. In the past most fpga engineers used the perspective of integer interpretation. Today and as result of software mindset invasion through their tools we have at times to shift between both concepts(integer or fractional).

if you discard 32 lsbs off 64 bits then in effect you are dividing the whole result by 2^32. Rounding will give nearest value. You don't round in the middle of bits at the imagined fractional point. In short you better think of 64 bits here as integer only(no decimal point). You round the 32 bits and it is the duty of receiving module to interpret the decimal point.

if your value is so small (relatively) that it only occupies 32 LSBs then you are going to get either 0 or 1 effect on the 32 MSBs. You then interpret that as 0.0 or 0.1/2^16 in Q16.16 format if next module is going to look at it that way.