Yes, that's right. The weight of the battens and boards should be knocked off the 25Kg/m2 but in reality, the trusses will carry considerably more than that without any problem. I'm just learning about the condensation issue, which seems to be of more concern than overloading, and i am beginning...
Condensation can be a problem, warm moist air will come up through the quilt, cooling as it goes, and hopefully reach the loft space before it reaches dew point and the moisture condenses back to liquid water, so it can vent away through the eaves gaps. By overboarding the quilt, the air is...
As a reundant truss designer, your roof trusses will have been designed to support your weight wandering around in the roof space on an occasional basis for maintenance etc, and also for storage of bits and bobs up to 250 newtons per square meter, (approx 25Kg). The size of the timber used will...
Easiest way is to get some trusses made. They will come with the calcs for building control too. They can make valley frames to overlay your existing roof that will take the new ridge into the old roof too.
What centres are you planning for the main 3.6m joists? If you are using 600 centres, then your trimmers are "replacing" one joist, if 400 centres, then "replacing" 2 joists. so doubling up on either side should be fine.
Do you have a sketch of what you trying to do? I am somewhat confused, how this is going together. Are you going to split the joists over the internal wall?