Can SCM cause mastoid process to be slightly enlarged. It's not sore not tender no ear infection. It's more razed up, rounder, than the other one?

Normal variant. This is likely a normal variant. Your mastoid processes do not have to be symmetric. Likely not related to your SCM. While one SCM may be larger than the other especially if spasming (e.g.torticollis) it should not affect your mastoid process.

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I have had a mastoid op about 26 yrs ago and I get really bad ear infections ever since but my problem is my jaw is really sore I can hardly chew.?

Jaw pain. Persistent pain around the angle of the jaw or in the jaw joint suggests the possibility of temporomandibular joint (TMJ) syndrome--painful inflammation of the jaw joint. The 2 major causes are advanced degenerative arthritis in the jaw joint itself, or excessive wear and tear, eg, in people who grind their teeth at night, or have chewing habits like gum, pencils, etc., and in chronic stress. Read more...