I always thought the bucket looked like it was full of ashes. So I took that as a symbol of what Ennis & Alma's relationship is like, compared to the fresh water in Ennis & Jack's. Sand would work too, I guess. It's the first time we see Ennis back to his regular life post-reunion, and the ashes/sand symbolize how dull and bereft it feels compared to his fun with Jack.
I really doubt Alma was seeing Monroe before the divorce. To me, Monroe seems like a fallback, a substitute (what IS the right idiom to describe Monroe's role? Sloppy seconds?). As symbolized by the electric knife, which I always assumed is meant, in part, to suggest a vibrator (is it only me who thinks this? or does everybody else think it's so obvious it's hardly worth mentioning, or what?). Like that's what Monroe is to Alma -- something you turn to when your man isn't available.