i might be worth finding out what roof pitch you have because as people have pointed out the roof pitch will determine what gauge you need. If your roof pitch is below 25 degrees you should have the battens spaced at 282 mm top to top giving you 100mm headlap on the tiles
I noticed all the colours you listed were concrete colours so its not a massive decision because in 10 -15 yrs most concrete colours fade and end up a brownish colour anyway
If you have clay plain tiles on your roof (10.5" x 6.5" Flat) the nearest looking match you will get which goes down to 15 degrees is a Sandtoft 20/20 & we sell a lot of these for this reason.
With a question like this your going to get everyones opinion not a definitive answer. So in my opinion Klober Permo Forte is the best has more re-inforcing than other felts. Most people pick Tyvek as it is what Pukka is to Pies- a brand name not necessarily the best
Bitumen is sold in kegs of 40 kg or in smaller blocks, depending on where you go.
I dont know how much everyone else charge but we usually sell it for £28 + V.A.T. (40kg Keg)
your gauge depends on your roof pitch, below 17.5 degrees 318mm top to top battens, 22.5 deg & over 343mm top to top battens. Straight out of redland brochure
Your only option is to cut the tile, but make sure its done properly to prevent water getting in behind velux. I would recommend a BDX collar for your velux which helps seal & insulate velux. We sell a lot of them for this reason
There is a slate which will go as low as 15 degrees called cambrian slate manufactured by Redland. However if you have lower than that there is a concrete interlocking tile made by the same company called a redland regent which will work on roof pitches as low as 12.5 degrees