I wouldn't leave a car in Merced and go by bus from there unless weather conditions are really bad. The road from Merced to Yosemite through Mariposa and El Portal largely follows the Merced River and is an easy, pleasant drive. Not a big highway, but easier than the road to the north that goes through Groveland or the one to the south through Fishcamp. Easier than the road into Sequoia, IMHO (the southern road from Sequoia to Three Rivers and Visalia is a slow one-- lots of switchbacks). In Yosemite, Tioga Pass is open right now. If it is somehow still open when you're there, I'd seriously consider going out that way, then down Hwy. 395. One of the most scenic drives in California. No worse than 198.

BTW, there is no Mojave NP. There is a Mojave reserve. Not that much you could see by regular car, as far as I know. Have a great trip.