Corrosion of rear subframe mounting and front suspension arm

Have just had my 2007 Prius MOT'd (compulsory annual check in the UK for cars over 3 years old).

It passed, but with the following advisory information:

1. Subframe mounting prescribed area is corroded but not considered excessive nearside rear [2.4.A.3]
2. Subframe mounting prescribed area is corroded but not considered excessive offside rear [2.4.A.3]
3. Suspension arm corroded but not seriously weakened nearside front [2.4.G.1]
4. Suspension arm corroded but not seriously weakened offside front [2.4.G.1]

Tester says nothing to worry about. Not sure why this has suddenly appeared this year - wasn't said last year at test by same tester.

Questions:
a. Presumably the Suspension Arm is replaceable, but is the rear subframe?
b. Presumably there's nothing much that can be done except for maybe cleaning it up and applying wayoyl or similar, but not sure how long that would last and sounds like an unpleasant job!
c. Anyone ever actually had these parts fail due to corrosion, and if so at what age and mileage? (Mine has done about 90,000.

 

Anyone?

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