AFAIK the only extra holes needed on the LWB were for the self tappers holding the trim down. Never had a SWB so can’t comment.I just bought the roof prep for a swim and you "need" to drill 3 more holes per rail and add in the bolt bracket to the inside of the van. I have yet to do this and bolted down using the 4 existing pre installed bolt holes . Easy enough job just haddent the time. I used tiger seal to pin down the plastic ends. To avoid drilling those self tappers in.
There should be rubber arround the holes in to rail to seal it. Mine are now so they are good Nd not squashed. A wee bit of silicon should help.
Part numbers for all the bits are on another of my threads.
If you use bolt down cross bars then each bar can take 100kg apparently. Still looking for c channel fitted bars but thule do them and a few others so far that I have found.
Yes Thule do feet and bars that will do 100kg
Be aware that all of the roof rack search sites offer you a foot pack that is quite high and it is possible to get ones 30mm lower. There is a thread about this elsewhere on the site.