Just my €0.02, from experience...
WF100 vs. "RG6" - WF100 is essentially a subset of RG6 coax, if it could be called a subset as RG6 has no strict rules on its dimensions or materials in the present day. In the UK, most RG6 isn't sold as using only copper for the metal, which is what makes WF100 stand out against "RG6" - at a cheap level RG6 is sold with a copper clad steel core wire with the shield and core only being aluminium. For the average installation in the UK of a single dish aimed at 28E this is unlikely to be an issue and over the average 10 to 15 metres length of the coax from LNB to receiver, the difference in signal loss will be very little, if any is actually noticeable. The biggest concern would be that the use of the higher resistive steel and aluminium metals in the coax can cause a larger DC voltage drop across the "RG6" coax compared to the all-copper WF100 over the same distance which could result in some LNBs not switching polarity correctly, causing "No signal" messages to appear etc. Beyond that, some "RG6" might have a pure copper core, but have say an aluminium foil shield covered by a fairly thin copper braid, and the dielectric can be either foam or hollowed "honeycomb" plastic.
On to the short "flat coax" joiners, I've used them a fair few times for both satellite and terrestrial use over the last number of years and are very useful in either temporary situations, or where drilling into a window frame or a brick wall is not an option. Whilst the theory mentioned by another poster involving a drop in signal levels because of the two joins in the flat cable is definitely possible, I've never noticed any difference myself in using a flat coax piece in marginal situations. The one thing I would say about them however is that these "flat coax" pieces are very fragile - ordinary coax can, depending on its materials and manufacturer, stand up to varying levels of abuse and weathering and while this "flat coax" hasn't given me grief due to weathering or UV exposure (though the usual caveat applies in waterproofing the external join of the flat coax connection, ideally with self amalgamating tape or maybe heatshrink), it will not take repeated window or door slams terribly well. If you need to use them, make sure they go through a window that is rarely opened (ideally never except maybe once or twice a year for cleaning) and when closing it, be gentle. The "flat coax" can fail very quickly thanks to even mild abuse through repeated crushing. In saying that, I had for several years used such a piece to allow a coax cable feed into my kitchen for terrestrial TV & Radio where I couldn't drill into anything at the time, and prior to that I once lived in a flat (third storey up) where the only aerial point was in the living room, and so to feed a split-off into my bedroom I used two pieces of flat coax at a window in each room that had an ordinary coax cable strung between them which did the trick - drilling through the wall into the bedroom was definitely not an option! So if you're still thinking of getting a "flat coax" piece, get at least two if not more. That way if one should quickly fail then it you will have a spare at least to quickly replace it with (weatherproofing & waterproofing not withstanding).