SCEcorp. Forecasts Earnings: Officials of SCEcorp., parent of Southern California Edison Co., said earnings for 1993 may fall slightly below recent analyst expectations of $1.69 a share, which would be a modest increase from the $1.66 a share earned in 1992. In addition, there is only a chance that SCE's Mission Energy unit will withdraw as a participant in the Carbon II electric generating project in Mexico, officials said. If Mission is forced to withdraw, it would lose $20 million on the project, mainly on development costs. SCE had counted on higher earnings from Mission to offset expected lower profit at Southern California Edison.