Don't be afraid of contacting these authors. Many authors I know would talk to a budding author about their experiences or at least answer a brief e-mail. It is best to double or triple space the text. Allow additional space between each page. If the subject matter requires special visual interpretation, a brief note in parentheses should suffice.
Editors look at hundreds, maybe thousands of such submissions.
Trust me, they can visualize what kind of illustrations your book needs. Unless your cousin lives next door to Steven Kellog, you'll be better off without illustrations. Part of the job of the publishing company is putting together the words of the author with the artwork of the illustrator. This is what editors and art directors get paid to do. By suggesting specific illustrations, your cousin's neighbor, may prejudice the editor against your manuscript.