I recommend this company to use RI or EVA instead of ROI and to control the investments separately using NPV and capital turnover measures.
The bonus should be based on the budgeted income level, the RI target
The problem with the inventory level can not be controlled with ROI management.
If the company change to RI/EVA it will be possible to to negotiate relevant inventory levels in the budget process. High inventory levels can also be managed with differentiated capital charges that will create high interest costs.
The best way to control operational tasks is to us nonfinancial measures such as inventory turnover.