Can P205/55R16 89H tires on my 2012 Honda Civic EX Coupe be stretched on 18x8.5 rims?

Can P205/55R16 89H tires on my 2012 Honda Civic EX Coupe be stretched on 18x8.5 rims? - 1

No. You have to get 18inch tires for an 18inch wheel.

Don't be silly… They are too small. And the tyres were never wide enough for 8.5 wheels

Physically impossible. Only an 18 diameter tire can be mounted on a 18 inch diameter wheel, regardless of tread width and sidewall aspect ratio.