The hike up along the Merced river canyon is beautiful and a perfect early season hike.
I don't find Merced Lake all that exciting and if you camp at the lake you are limited to the backpackers campground. For that reason I would only go as far as Echo Valley, a fabulous place to camp and explore. So here are a couple of variations on your itineraries one and two.
Itinerary one is a mellow trip along the Merced River
Day 1: up the Mist trail, then following the Merced River past LYV to Morain Dome "campground", 5.5 miles
Day 2: continue to Echo Valley, 5 miles
Day 3: go back the way you came except taking the JMT for that last part 12.5 miles mostly down hill
Itinerary two is more ambitious with higher milage, particularly on the first day
Day 1: to Echo Valley 11.5 miles
Day 2: take the high trail to Clouds Rest / JMT junction (there are two such junctions, take the southern one), fill up water here, go up to Clouds Rest, then back down a little ways to camp. 10 miles
Day 3: back down to JMT junction and then down JMT all the way to the valley. 8.5 miles