Child support/alimony/custody need help!

Divorce effective Oct 2006 in Goldsboro NC, joint custody with primary physical custody to her, very liberal visitation. child support and alimony agreed without court interference. In Aug of 2007 she decided she wanted to move to Missouri to live with boyfriend. After mulitple discussions with the children (then 13 and 15) the custody order was changed to allow for better visitation. Alimony $700.00 per month for 7 years after the effective date of the divorce, Child support $1300.00 per month. I flew the children here to nc 5 times per year at my expense, and flew there for a long weekend when there were more than 3 months between visits. In June 2008 she decided to move to south carolina (boyfriend did not work out), an oral agreement to allow for extra weekends with the girls since they were now only 4 hours away as long as I paid 100 percent of transportation costs.
After several years of uprooting and moving (3 different schools) the girls had had enough. In Jan 2009 the girls then 17 and 15 decided they were not going to live with her any longer. Jan 23rd of 2009 the girls moved back home with me. Since moving back they have spent less than 20 nights per year with her. She refused to sign a new custody agreement (as she did not want to be bound to required visitation dates). She claims now that they do not live with her she is going to school full time and only working part time. The short visits they do have together are only if the oldest child drives them both down to s.c. and I usually end up paying for their visits. I have also paid for transportation for the youngest to spend time with the ex’s parents in alabama. I still pay alimony (700 on the first of every month). My questions: Can I seek past child support for both children and future child support for the youngest who hasnt aged out. She claims she is going to school full time and only works part time if that. If she is able to work but doesnt, does that legally allow her to not pay support? I make 58000 to 60000 per year and we are scraping by, but as the children get older, our expenses have increased. When i use the child support calculator even when i use a very minamal income for her it still shows her requiring to pay. Can the alimony agreement be modified to collect child support? Thank you for any advice you can give.

You may seek child support, going forward. Any past support will be based on actual expenses you can demonstrate, rather than the guideline amount which is only awarded retroactively from the date you file for the same.
If she is not working in bad faith to avoid paying support income can be imputed to her in the amount she is capable of earning.
Alimony agreements are not modifiable.