I wouldn't accept that the 'undercarriage damage' was the cause of the
engine/radiator failure.  And if you don't know of some accident you had on
day 11 that could have caused the damage, then it may have been faulty
maintainence of the rental agency.
Since you're being blamed for a whole new radiator and engine, it would be
worthwhile to hire or have your insurance company inspect the damage and
make a diagnosis.  If the rental company won't let your representative
inspect the damage, then they shouldn't expect you to accept responsiblity
for it.
The fact that you went 10 days without a problem means something occurred on
day 11 that was the problem.

unfortunatly, jerry, i have replaced dozens of radiators in cobalt (and z
body malibu, same design) in the rental fleet im in charge of. the design
reminds me of a bucktoothed goalie without a facemask just waitin' for
the puck....the design lowers the radiator/condensor module to about 8"
above the ground and mounts it solely with a peg on both end caps poking
down thru 2 very light guage brackets...one high curb can push bracket
up, break off either peg and drain the cooling system within
seconds.....ive replaced only one engine from this condition, cobalt,
eco-tec.....customer was billed around $3,500....customer fumed but i had
pics and machine shop documentation. customers private ins. ended up
paying and custumer merely paid deduct............before writing a check
make sure to find out all your rights and responsibilitys.....your ins.
could cover it or maybe your CC............good luck, kjun