it could be that some where on your domestic hot and cold system the pipes are to close together allowing heat to transfer to cold from hot pipe or your cold pipe could be to near c/h pipes which will also allow for heat transfer
if you use option2 instead of using copper saddle you could use one of these 15mm straight wall plate adapter which would give a more ridged fix available from screw fix in gas fitting section or any plumbers merchants
buy a simular drain cock drain down heating remove spindle with square end also locking nut on end off drain cock. do the same with a new drain cock the new part will screw into old drain cock refill and test or just replace washer
check out one of these hudson reed chrome retainer bath waste & and plug and chain.E347 you may be able to fit the chrome waste outlet cover onto the over flow connection and use overflow pipe to connect to sink trap they cost about £8-50
will you be cutting a hole in the flat roof for soil pipe If you are you will need to check which way the ceiling joist run and what thickness they are as that could determine the position of soilpipe
you can do it with these fittings 22copper x 22copper x 1/2" female iron tee plus a 1/2"x1/4" brass bush most plumber merchants will stock these or toolstation
the 110 soil should fit you need to taper end of 110 pipe put some grease or washing up liquid on rubber seal and give a good push as it will be a tight fit but it will go