No one has mentioned that WVO can cause sticking rings. The increase in oil consumption may be caused by the gradual sticking of rings.
As you say that you are not using much WVO, just stop it all together for a few months and see if there is an improvement.
The clack clack at low revs could be injector noise. the loosening of injector hard lines is a good way to check on this.
Doing long distance h/way runs normally improves the injectors & quietens them.

Your idea of running the tank down very low & then filling with diesel is a good one.

__________________
Grumpy Old Diesel Owners Club group

I no longer question authority, I annoy authority. More effect, less effort....

1967 230-6 auto parts car. rust bucket.
1980 300D now parts car 800k miles
1984 300D 500k miles
1987 250td 160k miles English import
2001 jeep turbo diesel 130k miles
1998 jeep tdi ~ followed me home. Needs a turbo.
1968 Ford F750 truck. 6-354 diesel conversion.
Other toys ~J.D.,Cat & GM ~ mainly earth moving