When you find yourself wanting to use a common standardised connector for a non-standard purpose, stop. Don't do it. Pick a different connector, there are lots.
You will not be able to use standard USB cables for your design - you would have to make your own.
To answer your questions:
- are the Vbus pins inside a USB-C receptacle physically connected already or is this happening on the PCB pins?
Not in the receptical, all of the pins are wired out seperately. But they might be in a cable.
- Are the individual D- and D+ pins (for reverse polarity) A7 and B7 (A6 and B6 respectively) connected inside the connector?
Again, not in the receptical.
- Are the wires inside a USB-C cable for D+ and D- connected already (2 wires inside the sleeve) or are these 4 individual wires?
All of the cables I have seen have only one pair of cables (2 wires) and not two pairs (4 wires). They are ususally wired up to one side of the plug (e.g. A side). For USB applications it is then up to the device to work regardless of which side of the connector is wired up
- Same question goes for RX1/2 and TX1/2 (i.e.: is RX1+ physically the same wire as RX2+?)
That one is more complicated. There are at least four flavours of USB cable:
- Power only - These have just the power pins and no data lines (though some might, it's complicated).
- USB 2.0 only - These don't have any of the superspeed lines
- USB 3.x - These typically have only two pairs of superspeed lines, one for Rx1p/n and one for Tx1p/n.
- Thunderbolt compatible - These have four pairs of superspeed lines, seperate pairs for each of Rx1p/n, Rx2p/n, Tx1p/n, Tx2p/n
You also have to be careful of the fact that the Tx and Rx lines are crossed over in the cable (i.e. Tx at one end connects to Rx at the other end).
- Addition: J5 Heater should carry around 3-5A. Should I maybe use Bvus and GND to carry this load and "abuse" another pin for GND (no load)?
Don't try and use the GND lines for anything other than GND. There is no telling how they are wired up in the cable itself, they may connect to the sheild, they will likely all connect together.
Furthermore the VBus line for USB cables will not necessarily be able to handle 5A.
All USB-C cables (assuming they conform to spec) are required to be rated for 3A (20V 60W PD). There is an optional 100W PD mode (20V 5A), however cables are not required to support it, and those that do have active electronics to identify that they support the higher power rating.