After I have read your thread a couple of times I became a little confused over the DTC description your are getting on your Snap On tester.
Firstly I don't think the DTC description (G98) are interpreted correctly from the ECU data on your tester.
Why not !!! because when i look at Self-study Programme 465 where the system are described in details are there are no description of this sensor (G98), there are 3 sensors as you already have mentioned with the following names - exhaust gas temperature sender (G235 - G495 - G648).
The description you got from local volkswagen dealer regarding the diagnostic trouble code 15536 EGR temp sensor (G98) sounds more likely to be correct, that its a exhaust gas temperature sensor and not egr valve temp sensor.
I might help you with the interpretation of the voltage readings from the exhaust gas temperature sender (G235), if you read the last part of my case study regarding " How to measure a Bosch LSU 4.9 wideband lambda sensor " https://www.picoauto.com/support/post32289.html#p32289
where I have logged some live data readings from this sensor (G235). The case study are based on a Jetta 2.0 TDI with DPF (diesel partikel filter) which basic are the same as fitted on your Polo 1.2 TDCI system.
From your own experience of measuring these exhaust gas temperature and the result you were getting of 1 volt on all these sensor - would I say, that this are really low voltages readings.
There may be a link between the low voltage readings from exhaust gas temperature sensor (G235) and the EGR-system. If the egr valve system are hanging will there be some disruption of exhaust gas flow through the exhaust system.