The gas and pipework characteristics are - Biogas - 60% CH4, 40% CO2, traces of NOX, SOX & HCL

A butterfly valve which transmits a signal or can provide some sort of indication when used of whether it's fully closed or not fully closed.

Cnfirm or correct the understanding of the terms firetested and firesafe; A firetested valve must be operable after the test but need not be seal tight whereas a firesafe valve must fully seal.

This might be relevant. Without retainer indicates perhaps a direct metal to metal sealing with direct contact, again the definitions for HP butterfly valves might be significant.

Removal of valve from line requires removal of nuts and all studs from valve body and result in delays, one of the options is to change tapped holes on valve body to through hole by drilling,

Performing a cooling water system calculate in Excel. The system is being pressure balanced using butterfly valves.

Most free or very low cost sizing and selection software given away by a vendor is generic by nature and won't contain all the selections that their in-house propriatary sizing software will give them

You will either have some special constructed regulating composed disc, cage type or similar, to give support for a complicated and large pressure flow and drop.