AFter reading this whole thread, Is it necessary to use a relay to disengage the clutch for remote start?
No but depends on the alarm. With a DEI the blue status wire can be wired directly to the Green/Yellow wire on the clutch safety switch. What this car wire is looking for is a ground "only during remote start". So any alarm wire that does that is fine. However if you intend to get the rear defroster working you may need that wire.
In regard to the tach wire behind the cluster all i can say it works fine for me with a DEI Python. The only way to tell is to use it but recommend a solder joint and not a T tap.