Looking around my house I have numerous pictures and signs hung off a single pin or double pin hook as per pic on standard plastered brickwalls and plastered stud walls and in over 50 years have never hit a wire/pipe/etc etc . and also used to hang larger objects with with a screw and plug...
75 x 150 will enable you to have snow loading of 0.75kn/m2 ,dead load of 0.5kn/m2 and have deflection of 17mm which is 3mm above recommended max of 14mm but its got to be quite a large snow fall for that and down south snow loading can be reduced slightly
a) no way
b) at 200 centres and assuming the dead load is not greater than 2.5kn/m2 and snow loading does not exceed 0.4kn/m2 as opposed to standard 0.75kn/m2 it just comes within acceptable deflection limit of 17mm
c)the span tables are the span tables
Just running things through theoretically. A very very heavy rainfall would be classed as 50mm of rain/hr which for a roof of 120m2 would mean a total of 6m/3. 6m3/hr = 1.7l/s . The head required to push 1.07 l/s down a 62mm ID cast iron pipe is a lot less than 1m so your downpipe is not going...
Must admit in all the tyres I have fitted over the last 50 years I have never gone and looked at the inside of one before the fitter fitted it, usually just sat in the reception area twiddling my thumbs.
whilst not having direct involvement with domestic 150mm dia drainage runs I worked in the main drainage/sewage department of ST for 40+ years and would expect a competent builder to be totally acceptable to carry out such work.