Carl the Engineer

Wife,
From what I read, you state your husband is 70% IU, which means he is being paid at the 100% rate.  Is he P&T?.  What are his individual ratings and for what.  What exactly are you after?
As far as being service connected for a TBI (one is enough, more don't matter), yes a buddy statement will help as well as his statement.  If nothing else, file for the TBI now and worry about the early date later, just in case you don't get it, you have the current date.  File a claim.
I would think his unit (attached medical unit) would have anything the medic would have had.  I can't beleive he kept all his records on his person at all times.  Did your husband recieve 3 TBI's on the same day the medic got killed, if you catch my drift.  I would check with his unit, and the medical unit that was attached to it.  In writting.  I actually contacted my Senator and had him to a request for my medical records which I could not get on my own.  It worked for me.
Also, yes, file for everything that occurred while he was in service and in addtion, file for any new conditions that can be attributed to his current service connected disabilities, secondary to the current ailment.  Secondaries are easier as you only need to prove the original condition caused the new condition.  You don't have to prove it occurred while in service, etc.
Good luck,
Hamslice