>Can anyone recommend any good study/studies on the meanings of vidhi-? The use in MBh 12.290.2 appears to connote "method" but I suspect its scope is wider here.

On the one hand, in Vedic sacrificial literature, such as the Brahmanas and Srautasutras, the noun vidhi denotes sentences written in the optative mood, giving direction on how to perform Vedic ritual, e.g., "the mantra should be repeated three times." You can read about this usage in Jan Gonda's book "Vedic Literature (Samhitas and Brahmanas)" in the series called A History of Indian Literature, vol. 1.1 (Wiesbaden: Otto Harrassowitz, 1975), pp. 324 and 340. See also Gonda's book "The Ritual Sutras", likewise in the series A History of Indian Literature, vol. 1.2 (Wiesbaden: Otto Harrassowitz, 1977), pp. 497-498.

On the other hand, there exist numerous texts in later Indian literature of Hinduism, Buddhism, and Jainism with the word vidhi in the text's title, where vidhi simply means "ritual", for example Visnu-puja-vidhi ("Ritual for venerating Visnu"), Abhiseka-vidhi ("Initiation Ritual"), or Vivaha-vidhi ("Wedding Ritual"). I have not seen any broader study or description of the genre, but for examples of such text-titles try to do a search for vidhi on the Nepal-German Manuscript Cataloging Project's homepage (16.619 manuscripts containing the word vidhi in the title):

http://catalogue.ngmcp.uni-hamburg.de/titlelist with a search for %vidhi% (the percentage marks create a wildcard search before and after the search word).