Until the full content is released most information can be neither confirmed or denied. I would assume Greivous is in season six because in the clovis arc preview the battle of coruscant appears and General Grievous is a major part of that arc. Also because there are a maximim possibleity of 13 episodes for season 6 part of season six could take place when General Greivous is in charge of the Confederacy or even Nute Gunray. It has also been confirmed that Ahsoka will appeare in the soon to fail upcomeing TV series Rebles. None the less I can assure you Grievous will be in season 6.
Well on Jan.17 an artical came out on JediNews.net that on Feb.15 the content will air in Germany so the date it will air in the U.S is likely the 15th or soon after or before. Rumors have been said that it will be released through media devices like IPAD video store or AmazonVideo. It has also been confirmed that there will be many battles in the 13 episode bounus content. I did give descriptions for the episodes not long ago and listed the sources but like the 9/10 things I post here it was revocked?!. If it would air on TV it would most likely be a Disney owned channel like DisneyXD. But as of today the 9th the Bounus content is not to far away. Also the content will be released arc by arc once a week. But to answer your last question if you keep checking for updates you won't miss it.
P.S to find the website with decriptions and names for the episodes of the bounus content look up Star Wars the Clone Wars season 6, then click on the result on the website Metro.u.co then they should have a link to theForce.net.