"Is it then likely that only Mount could defeat Boustany?"
I would have to believe that it would be extremely difficult for a black state senator such as Cravins to beat a Cajun Republican such as Boustany in a district that gave Bush 55% (IIRC) in 2000. But a Methodist woman from all the way over in Lake Charles likely wouldn't run very strongly in Laffayette either. Maybe one of the Louisianans FReepers can give us a more informed opinion on the matter.
"Could it be that Thibodeaux will poll enough Lafayette Parish votes to keep Boustany out of the general election?"
I certainly hope not, and if it is clear that Boustany is well ahead of Thibodeaux I think someone has to talk to Thibodeaux and convince him to drop out and endorse Boustany. We can't risk another all-Democrat run-off in the district like the one they had in 1996 (when Chris John got to face a Lake Charles Democrat instead of Thibodeaux, who was less than 100 votes from the second run-off spot).
The runoff will be decided in the Crowley area in the central part of the district. This is essentially neutral territory with no hometown candidate, but it identifies with Lafayette far more than Lake Charles.