As MHC says, your diagram shows a real lash-up of a solution, which is likely to perform badly. You want one filter at the master socket and no others.
I'm not even clear whether we're attempting to reuse an existing cable through the cavity wall or not. If so, how many wires does the existing cable contain, what colours are they, and do they appear to be twisted together in pairs?
Are the only places you need a phone socket next to the FTTC modem, as in your diagram, and possibly at the master?
If you are installing new cable through the wall, the way to draw attention to the ends of that cable and be likely to damage existing decoration is to mess around letting back boxes into the wall and cover them with blanking plates.
Depending on whether you extend the VDSL or leave the modem where it was and connect modem to router over an Ethernet cable, you need one or two runs of solid core Category 5e cable. This will run easily in some mini trunking - just drill from the back of the trunking and fit an end cap to the trunking. If you subsequently redecorate, you can chase the cable into the wall and fill over the top.