So that means you can't use the stack for anything else but the unvented
No, not the stack just the waste pipes. It just states we need to demonstrated that the soil stack is capable of safety withstanding temp of discharge, not sure how was are supposed to do that but I normally say if it is vertical, clipped & vented directly so that there is air flow it should be OK.
Also that could be said for a normal dedicated line for the unvented terminal end could become blocked etc
Yes but you must admit teeing in to a waste pipe increases the risk dramatically.
Also there are a lot of 30 plus l undersink water heaters teed into drain lines that manufacturer instructions and building regs that are happy / acceptable with it
Not sure what you mean, I have not seen them but as we know two wrongs don't make it right & policing is weak. I am just quoting the Regulations Approved Document as I see it.
Also haven't you come across high temp solvent waste systems uses a different high temp glue to the normal stuff.
Can't remember but does it comply with BS7291-1,2&3:2006 if so fine it could be used. The problem is by stating that a solvent weld system acceptable the uneducated may be lead to believe that all S/W systems are OK.