My apologies for delay in submitting my replies and joining everyone in this discussion. I wasn't very well the last few days. I will do my best to keep the future due dates!
1. Employees' lost loyalty to and trust in the company. As Misuzu-san mentioned, I think the management team should have responded earlier to the Eastern Division's issue about the compensation structure, even if they couldn't change the scheme right away, or they needed more time to look into this matter. The employees of Eastern Division had hope at first after their initial discussion with Tanaka-san, and must have felt that they were being ignored by the company, and that their concern was not taken seriously. Therefore, they got dis-engaged, and lost loyalty to the company.
2. It is important for the management to show that the company cares about its employees, and I think HR (who didn't appear in this caselet) could have taken a role to support Tanaka-san's voice to make it heard more, or to convince the management to prioritize this matter over other issues by stressing the critical situation. As Rie-san mentioned, the company will have to look into the matter holistically. The current pay structure may not be the only issue, but the management team should at least show an attitude that they heard the employees' voices and are examining the matter, and perhaps set a time frame for their reply or show a transparent decision making process, if possible.
3. If I was in Tanaka-san's shoes, as a first step, I would closely work with HR to collect feedback from the other divisions regarding the company's pay scheme and, if possible, also about the competitors' comp scheme information. If the competitors are also keen to adopt, or are adopting, a merit-based pay scheme, I would suggest the company seriously considers a merit-based pay scheme, to retain good talent and keep competitive in the market. It may depend on the organization, but I think employees are more or less interested in being recognized for their effort in the way of a monetary reward, especially the sales rep groups. Of course, the management should look at the company's financial aspect and impact of any change when making a final decision, but they should also strive to understand what motivates their employees and to find out how the company can retain passionate and loyal employees.