A defendant should never “plea guilty” because “plea” is not a verb. The infinitive is “to plead.” The past tense is “pleaded” or “pled.” Which you use is a matter of personal preference, either yours or your readers’, but the Oxford English Dictionary and Garner’s Modern American Usage both prefer “pleaded.”
A case should not under any circumstances “be plead,” but it might be pled (or pleaded) if necessary.
(I wrote about this seven years ago. I’m probably the only one who remembers, other than Justice Jim Sharp, who in 2012 left a comment threatening to use “plead” as the past tense in a footnote. I seem to have (aided by the authority of OED and GMAU) prevailed on Justice Sharp, who used “pled” in opinions before his comment, and “pleaded” after.)