The car is in Scotland so has had lashings of salt thrown at it. Possibly an overzealous tester with all those advisories. However it obviously has seriously corroded rear sills and chassis legs.
As I have flippently said in the past buy a MX5 south of Derby and used as a second car.
The weird thing that disagrees with what you have said and what you have said it is what I advise people, is that some MK3’s from well down south are in a very sad state re rust and a good number up here around 2005 to 2007 are Ok ish.
These MK3’s up here are used all year in Aberdeenshire and we use loads of salt.
During the three years I existed in Blackpool I would try and lift myself out of the torpor by driving home along the seafront. Slightest bit of wind and my car was treated to a salt bath. Obviously that’s an extreme case but the nearer the coast you are…
My '98 Mk2 came from the south coast (Bournemouth, can’t get much further south) but hasn’t rotted “below decks”.
The likely main reason was that it didn’t get driven much (only 30,000 miles until two years ago) but tbh, I don’t think the local seagulls had done the paintwork much good!
Maybe this is a lesson for all MX5 owners to heed. When you get the car jack it up as high as you can. Go buy a big tin of waxoil & underseal. A wire brush & go to town on it. All MX5,s are prone to rust. I recently had a 2007 model in the workshop with 24000 miles. All the tubular steel arms/bars etc were rusty. After treating like explained above it looked really good & sorted for a good few years. I have a 2009 sport tech & the MOT tester said come & have a look at this. The underside looked like it had been teated by a pro as even the nooks & crannies were coated with undeseal. Looked like mayby Zymoil. The other problem as many know is the drain holes getting blocked & holding water especially the rear sill area. The MX5 is a brilliant car only to be let down by Mazda not treating them from new.
P.S if you don,t want to buy the waxoyl spray lance etc they sell it in aerosol tins to get into the hidden nooks.
No i do not work for anyone i,m retired. Just very fond of the Marque. Used to have a few MGB,s way back & they were terrible.
I find it suspicious that all 4 lower wishbones, inside of 12 months, went from corroded but not seriously weakened to being labeled dangerous. I know when Mazda changed the pressing used for the NB wishbones , that made them more prone to entrapment of moisture, and rotting, but this does seem odd/unlucky.