Hi,
I followed your calculations just until the point where you started to use lots of numbers and words
Unless you are in a hurry to decide I would experiment a little. Sounds like you have been looking into how to drive the Transporter efficiently if you are doing better than the official figures and others with the same vehicle. The Transporter has a lot of advantages, especially space. If you use it full all the time then you need to weight that against the cost of fuel for it. If its empty then the extra compared to a car is kind of wasted.
Still if you haven't, check out the tips page and the other TDI related threads. For example are their small eco mods you can made to the Transporter - e.g. grill blocks, or folding in the mirrors when on the motorways and using those small, convex mirror things - stuff like that. Even the tyre pressure mod for better glides.
For biking and camping / outdoor stuff though I would think the Transporter is a winner. I'm considering a van for my next vehicle because of this - the back of my Skoda is scratched like mad from bide racks being taken on and off, plus your bike isn't as secure on the outside even with locks.
For info the Octavia (my wife has one) is frustratingly just two small to get an adult mountain bike in easily (mines an old Peugeot - probably way out of date - did I mention my portlyness
). And when it is in there isn't room for the rear seats to stay, even the single split one. And of course when it is in it is on display - another win for the Transporter.
Ski stuff in the Octavia will need a roof rack which is horrid for aero. You can use a roof box instead of course.
There is of course the other option which sounds wrong at first however how about a second vehicle, maybe a smaller van or car when you don't need the space ? It depends on if you have the space to store one when you aren't using it, plus you have double tax, insurance etc. Multi car insurance can work for the latter and only taxing the vehicle you use can work for the former but then you have to keep the one you aren't using off the road, and in some areas of the UK even out of sight.
My point about the flywheel is to check it out - listen for a change of note or rumbling when you put the clutch in or out. It may mean something, or it may mean absolutely nothing. The issue is that when the clutch goes its worth changing the flywheel at the same time as its a pig.
DMFs are expensive and the time required is also expensive. The local Skoda dealer near me replaces DMFs with SMFs for taxi use as they don't break. Lots of people will say they are totally reliable and others not. My spanner people (VAG specialists) say they have had them go on vehicles with mileages between 10K and 150K miles unpredictably but they do replace them like with like. SMFs are more noisy in some cases (e.g. mine rattles) but in others not.
Either way good luck.