This thread seems to be trying to define bluegrass at the moment rather than looking at its roots. Another old thread, seeking roots (Celtic Roots of Bluegrass Sought, 22505) has been revived as well. Just glancing through it, one avenue suggested is to look at the music itself and its formation.

Guest Banjo Johnny (18 June 00) said, "one of the sources of Bluegrass is our "mountain style", and it shows one unique feature with what we think of as Irish/Scottish music, that is, the "flatted Seven chord...."
But if you pick up a volume on Negro spirituals, the following, or similar, is found (John W. Work, American Negro Songs and Spirituals, p. 27): "Of much interest are the scales of the Negro employed in the spirituals." ----- "But there were employed notes foreign to the conventional major and minor scales with such frequency as to justify their being regarded as distinct. The most common of these are the "flatted third" (the feature note of the blues) and the "flatted seventh." (The latter note prominent in "Roll, Jordan, Roll," etc.
From this, I would guess that some of these features were developed independently by several cultures, and are not a reliable method of defining roots unless other factors are brought in. The alternative would be to accept that Irish/Scottish and Negro music have roots in common.
Rather difficult to put a fence around bluegrass, isn't it?