First, you may have already heard this, but class 3 in Colorado (at least per Roach) is easier than class 3 in California.
The trail below LBSL crosses the stream twice. You can normally carefully hop across the lower crossing. You should be able to make the upper crossing without wading, but there is water splashing from a small waterfall, so you probably won’t make it across totally dry. By the time you get there, the snow bridges and the avalanche danger will probably be gone.
You will have to cross the creek (N to S) at the outlet of LBSL. You may have to do some creative rock hopping, but you should be able to make it without wading. If you do have to wade, it should be easy. It’s wide at the outlet, so the water is neither deep nor fast.
I’ve always dayhiked or camped at Iceberg Lake, so I’ve never camped at UBSL. A three-season tent should be fine unless the forecast calls for high winds or snow.
Adding Russell before you hike out makes perfect sense. The hike up to the Russell – Carillon saddle will be on a moderate sandy slope. It’s south facing, so there shouldn’t be much snow. The route from the saddle to Russell will probably have some patches of snow that you can navigate across or around.
BTW, if you haven’t done Muir, it’s not too hard to hike down the main trail, take a short detour to Muir, hike back to Whitney, and then descend the Mountaineers’ Route. The detour to Muir has taken me as little as two hours, pushing really hard.
