As you have discovered, mortise lock-sets are very tricky to get right and very easy to get wrong.
Mike's suggestion is your best bet assuming you can rent or borrow one, purchasing it would be cost prohibitive as it sells for over $1000.00 US new.
You might be able to use your router to remove the bulk of the material but I would recommend clamping some 2" blocks on either side of the door flush with the edge to give the router some added support. Trying to balance the router on the narrow edge of the door would probably end in disaster.
Making a template out of MDF and using a template guide would be even better, and with 6 doors to do, probably worth the time to make it.
"The difficult we do immediately... The impossible takes a little longer" - Grandpa
Don't worry about nothin', aint nothin' gonna turn out right anyway" - Dad