+    if [ $dev_removed == 1 ]; then
+	umount $SCRATCH_MNT
+        _devmgt_add "${DEVHTL}"
+    fi

messy indendation.

+	btrfs filesystem balance $SCRATCH_MNT || _fail "balance failed"
+	#btrfs filesystem show $SCRATCH_DEV 2>&1 | egrep devid |awk '{
+	#	if ( $6 == "0.00" ) { exit 1 }
+	#}' || _fail "test6_add... failed"

what is the point of the commented out code?

otherwise looks good to me.