First off, since the cbi is an investigative body, it is inevitable that new identities and events will come to light as the inquiry goes on. This is how they keep track of their progress. The same could apply to Jagan's name in the supplemental counter that the cbi filed. The cbi hasn't even contacted jagan for an inquiry; they've only mentioned his name in passing. If jagan launches a slander lawsuit at this time, it will simply irritate the cbi and put them on the defensive.
An investigating agency has to go forward with fresh claims and allegations in order to function. For this reason, bringing defamation lawsuits will put an end to their operations. The judgements of the courts will all be consistent with one another. Simply because the cbi included their names throughout the inquiry, they might not even consider such counter charges.