DPF at 90% clogging is perfectly normal. The important value is what it is just after a regeneration.
It could easily rise to 120% before it regenerates. Mine did, and it has a huge hole all the way through it.
Clogging % is just a time proxy value, it rises in most acases at roughly 5% per hour regardless of how the car is driven. I did a lot of work monitoring mine.
If allowed to rise by itself the car will attempt to regenerate the dpf and chances are the clogging % value will drop quite a lot, but, it will immediately start to climb at about 5% per hour again. That's just what happens. Clogging % doesn't have a direct relationship to soot loading.
To check for a blocked dpf the mechanic should ignore the clogging % value and instead should be reading the differential pressure sensor output. If it is less than 70mbar at 3000rpm it is probably in useable condition. (All those motorway miles you do definitely help it)
I don't think the car can go into limp home mode without causing a fault code to be produced into the mode 10 permanent logs or without bringing the MIL light on.
If it were my car I would read the differential pressure sensor output and if it is in the normal range then block the EGR valve. If you don't have the diagnostic tools to check the DPS output you can block the EGR yourself to see what happens
One of the best ways to spend money on your car is to buy an ELM327 OBD interface (if yoiu have a laptop) and download the free version of MultiECUScan. Yoiuc an do this sort of thing for yourself, you can read and clear fault codes easily too, all without fear of causing any permanent damage or making any lasting changes. They're only £15...and santa is coming soon