Have any other officers and senior NCOS come forward to testified on their behalf and if not, why not (if you know)?
Great question.
Girouard's lieutenant testified on his behalf in the Article 32, which I have up for your reading pleasure at my site. I warn you, it's over 900 pages, but I've highlighted certain parts of it to ease the pain, at least in the first 200-300 pages. His LT said that there was no one he trusted more in the field.
Girouard himself is an E-6, at the age of 24. His resume is longer than many officers, and there is a long line of fellow soldiers who served under, below, and with him who are anxious to testify as to his integrity, skill, and professionalism.