I don't know if this goes on time, but it might help in the future.
If I understand you correctly you have problems routing all your number to one single voicemail through the 1st alt. form. What happens is that when someone dials a number and it routes to the 1st alt. it always follows the route of that number and not of the number that you put in the 1st alt.
What I usually do, I create an ACD path. With ACD you can "recreate" the 1st alt. form.
Create an Agent Skill group that allows queueing with no agents logged in. Then create a path assignment with interflow enabled (interflow time 01 sec.) to the number that the caller will hear the voicemail message. As last put in the 1st alt. form the number of the ACD path to all the numbers that you want to forward to the same voicemail.
I hope this can help you out.