Capacitance is a function of the spacing of the conductors and the dielectric constant (aka relative permittivity) of the insulating materials. All other things being equal, the capacitance between two wires will be proportional to the dielectric constant of the insulator.
A vacuum (and dry air) has a dielectric constant of 1. Insulating plastics have dielectric constants in around 2-4 generally.
The thicker the insulation, the wider spaced the wires will be, and thus the lower the capacitance per linear unit, again, all other things being equal.
You can approximate the capacitance of an unshielded twisted pair by:
C(pF/ft) = \$\frac{2.2\epsilon }{log_{10}(\frac{1.3D}{f d})}\$
Where
D is the diameter of of the wire including insulation (inches)
d is the conductor diameter (inches)
\$\epsilon\$ is the dielectric constant of the insulation
f is the stranding factor (about 1)
Here's another reference that gives a derivation of a similar approximation.
The resistance of the conductors does not directly affect the capacitance but it will have other effects on the performance, especially when the lengths are long. Look up the Heaviside condition for more on that.
Yes, use twisted pair. Failing to do so for a differential signal is self-defeating. Don't forget to include 120-ohm termination resistors at each end of the transmission line.
The less connectors in any transmission system the better. Connectors are almost guaranteed to present an impedance discontinuity, and hence will cause reflections.
Transmission line stubs of any length are also a source of reflections and (if I'm reading your description correctly) you have transmission line stubs of several metres. The longer the stub, the worse the impact of the reflections.
Reflections are bad because they can cause destructive interference, i.e. they can corrupt any transmitted data.
I'd use a longer primary (twisted pair) cable, less connectors and shorter stubs. Run the primary cable as close to each node as you can, even if it means that the cable has to be longer.
Best Answer
The specification you're refering to must be interpreted in the following way:
Now take a look at this diagram:
Source of image: this paper. Ignore capacitances \$C_1\$, \$C_2\$, \$C_E\$, and assume that \$C_{S1}=C_{S2}\$.
From the diagram we can see that, because you're shorting one of the conductor to shield, the capacitance measured must be then the parallel of the conductor-to-conductor capacitance \$C_{12}\$ and the conductor-to-shield capacitance \$C_{S1}\$.
Capacitances in parallel add together, so we can calculate the conductor-to-shield capacitance \$C_{S1}\$ from the specification, just subtracting. In this case, the conductor-to shield capacitance \$C_{S1}\$ is just 100 pF/ft - 60 pF/ft = 40 pF/ft. That's a 40 pF/ft from each conductor to shield (two capacitances) in your model. Exactly like analogsystemsrf has outlined in his answer.
This is only for the capacitance. If you want a complete lumped model don't forget the rest of parameters (inductance and resistance per length unit) that you can find in the datasheet.