Confused by colloquialism...

[orbiter]

I have heard people use "고만하다" in place of 그만하다, e.g. "미친소리 그만해라". But "고만하다" actually means to be "alike, similar"; so is the person really trying to say 그만하다 in an accent or is there another meaning to 고만하다?

And I often read shortened expressions like "그렇담 어떻 게 해야해요?" or "오래 살았슴 좋겠어". Do people really speak like that to each other?

미리 고마워요!!

[manyakumi]

I have heard people use "고만하다" in place of 그만하다, e.g. "미친소리 그만해라". But "고만하다" actually means to be "alike, similar"; so is the person really trying to say 그만하다 in an accent or is there another meaning to 고만하다?

There's no other meaning.
Just to sound easy or somewhat friendly.

And I often read shortened expressions like "그렇담 어떻 게 해야해요?" or "오래 살았슴 좋겠어". Do people really speak like that to each other?

Yup.
The sound changes as...
그렇다면 : 그러타면 -> 그러타믄 -> 그러탐
살았으면 : 사라쓰면 -> 사라쓰믄 -> 사라씀
You can hear tons of contractions like this if you were here in Korea.