http://www.hiprom.com/Pages/Products/1788_Range/1788HP-EN2PA-R/files/Document%20Publish/ProfibusPA%20Physical%20Layer%20Network%20Design%20Guidelines%20v1.00.01.pdf

According to section 12.8.6 of IEC61158-2 (Physical Layer Specification) the terminator impedance will be equal to a 100Ω resistor in series with a 1μF capacitor

So basically the terminator acts as an impedance and the value cannot easily be measured with a multimeter for example. An active terninator will eliminate reflexions onthe PA line so transmissions will not be overrun and destroyed.
The effect of the loss of termination will be immediately recognized by the DP/PA link and an alarm will be generated (SF on the IM 153-2 and on the PLC, diagnostic buffer messages).

2) Power to the PAinstrument is provided over the bus.On the fly replacement will depend on the type of junction box you use, If it has integrated short-circvuit protection (we generally used JBs from MTL for that purpose, even though they are branded for Foundation Fieldbus, the bus physics are the same as for PA) then you simply disconnect the cable at the JB and change the instrument, then reconnect.
If you JB has no individual short-circuit protection, the I suggest to use (or plan)a plant shutdown to remove power from the DP/PA link and then simply change the unpowered instrument.

3) In the type of JB you show, you have a pass-through connectionfor the trunk line and several spurlines connected to instruments.
We always kept at least one spare spurline connector so a breakdown of the spurline connector in the JB would only require changing the wiring of the defective to the spare. Our MTL JBs had a defect LED on each connector to help us see the status of each one.
If however the pass-through trunk connectors were to become faulty then you would lose the whole line, because of termination loss.

4) See 3)

5) See 2)

6) See 1): termination loss willbring the whole PA line down.

Hope this helps,
Daniel Chartier