The last lead joint I wiped was about 35 years ago, on a 2" water main to a school at 7.00pm down a hole on a November night throwing it down with rain with let by on the valve. As far as I know it is still ok to this day.
The gas meter is capable of providing the likely demand. The gas installation pipe will have to be sized according to the pressure drop over the run, which can be calculated by the gas safe registered engineer. The gas installation pipe must not be ducted under foundations. You can bring it up...
As it is intermittent and at peak times my money is on poor design sucking a trap out somewhere. You can get anti siphon traps as a cheap fix for poor design.
As long as there is no risk of contamination of water supply, then soil stack is a good method, as the water is brackish and therefore should not discharge into the land course.
Water seals not only on wc, but on all water traps on the waste system. May be poor design of waste system sucking the water out of a trap. Look at anti-siphon traps or venting the waste system
With a bit of digging I can probably find the pre- 2000 BS5440 standard and part J building regs. But in all of my career I have known it as a no, to terminate a flue across a boundary; which goes back to 1970's.