Insulation resistance of autotransformer

https://www.usbr.gov/tsc/techreferences/mands/mands-pdfs/Trnsfrmr.pdf

https://electrical-engineering-portal.com/measurement-of-insulation-resistance-2

A1) Yes. The original equipment manufacturer can. Get on the phone, and ask for advice.

A2) Recommended, yes. Moisture will affect the reading.

A3) The original equipment manufacturer's advice needs to be followed.

Always use the phone as one can wait quicker. Do let the forum know how the phone call went.

Disconnect the Neutral-to-Earth connection and check the IR value by applying voltage at any of the six terminals (seven if we include neutral as well).

There is no need to remove / drain the transformer oil.

If the IR value is low, further steps will be required such as processing/filtering of transformer oil to remove moisture from oil as well as from winding insulation.

but what are six terminals?

The original equipment manufacturer can answer that question over a phone call. Are phones not available at that location, given that there are more on the planet now than there have ever been? This forum cannot offer training in how to use one.

Before carrying out any <...test...>, it is important to establish where the boundary between pass and fail is.