Yes Dm7 is a substitute as its the upper structured triad of Bbm7#11 Its just Bbm7#11 without a root. Nothing to play along with there.

You don't seem to grasp tonic prolongation and that is evident by your suggestion dm7 would sub for Fmaj7. You are only seeing things on a chord by chord basis. You are not looking at the big picture.

You are resolving on the beginning of the cycle back to Gm to make your point.

Listen I'm not going to bicker with you because with music theory you can make a case for anything and I don't have time for that.

When I did my M.A. my analysis professor (who was brilliant) really loved keeping things simple. He said if you ever wanna know what key something is in listen to the last chord. 9 times out of 10 that's your key center.

https://youtu.be/XUSGULIdxXc?t=2m12s

What chord does it end on?????? Gm? No because the progression ends on Bbmaj7 the root, not the Gm that started the progression ;)

The sad thing is that all you need to do is use your ears. This is one of those cases where theory becomes an obstacle. You are not using it as a tool. You are trying to use it as a weapon and its blowing up in your face. Am I right in assuming you are guitar player?

Your last statement is the problem with guitar players and their perception on modality. Guitarists learn modes as a means of soloing. They figure oh I am in Bb I will solo on the F major scale and it will be lydian.

They are not the same scale. You are missing the point on tonal centers. They share the same key signature. They are not the same scale.

Also as a final note (pardon the pun) If your analysis has your 5 chord as a minor. You fucked something up ;) Blues is a good example of a dorion tonality with V7.

Vm7 lol. You So Silly.