It is times like this, that I like direct quotes. In doing a google search I could not find direct quotes on the use of the word "consider" and "considering" in regard to Mark being not a suspect. I did find this:
From Nov. 29th -
"The Sheriff's Office is not calling Mark Redwine a suspect," Dan Bender, La Plata County Sheriff's Office Public Information Officer, said. "However, since that house was the last place Dylan was seen, it is only prudent to do a more thorough search of that house and property for any information that can help direct us to Dylan."
http://www.9news.com/news/article/302104/339/Feds-search-missing-teens-dads-home
IMO they are not going to call him a suspect if he is co-operating & talking and maybe not even then. Once they call him a suspect, it is my understanding he is treated differently with regard to rights (but I am not sure how that applies). Considering & calling are two very different things IMO.