Hi KayN - Would you please post the URL for the WWII free offer on Ancestry.com? I could only bring up US records. (Or did you mean to write WWI for the Subject? If so please post that URL on Ancestry.com anyway, for others to use the free offer?)
I subscribe to Ancestry.co.uk but they often exclude access to UK records which they make available on Ancestry.com.
WWI Service records have the records of John Archibald Kinvig b.abt 1895, residence 2 Stanley View, Douglas Iom. 13 images plus 2nd entry 2 images. Aged 20 in 1915, Baker, next-of-kin father W(m?) Kinvig 2 Stanley View Douglas.

You wrote that Mary's record of your Herbert Kinvig was correct - but not WWII - died in 1918.

Sue