Is this rust easily treatable? NC2.5

Welcome to the club and MX-5 ownership.

Best advice to anyone buying any MX-5, even brand new, particularly if they envisage long ownership is to take it to a reputable specialist for an underside with cavity inspection and get a quote for taking the whole underside back to bare metal and getting the cavities protected.

Yours does not look particularly bad, as you aleady indicated slapping stuff on top will only accelerate the process and personally I’d avoid what your dealer offered even if it’s free, 1 or 2 hours is unlikely to get the job done thoroughly and I am guessing your dealer is neither Mazda or an MX-5 specialist.

Regarding the clunking this old thread might give you some pointers :- MX5 2011 NC Clunking rear suspension

Wishing you many smiles per mile, zoom zoom !