Get permit, rest/nap, dinner at 8:30 pm and hit the trail head at 11pm.
You're actually not supposed to start until after midnight with the day permit. Although, if you don't enter the Whitney Zone (which starts right above Lone Pine Lake) until after Midnight, I think technically you're okay.
1. I was reading about the "old" trail vs. the "new". where do the trails diverge? Which one is recommended? Is 1 longer than the other?
The new trail is a little longer, but not as steep. People were suggesting it to avoid a high river crossing earlier in the year, but that crossing is no problem now. I'd recommend sticking to the new trail that starts below the Whitney Portal store area. The old one is up near the upper parking lot (search this forum for details, pictures, and GPS coordinates if you really want to find it).
2. We plan to carry 4L of water - 3L in bladder and 1L in water bottles. Is that too much, too less?
Per person or for all 5 of you?
I personally don't think I could make it to the summit and back to the Portal with only 4 liters.
Every person is different, but altitude will make you drink more.
Are you planning on getting water from the streams/springs as you go? You can save a LOT of weight by doing that. Just need a good filter or UV pen to treat the water (and some diehards don't even bother... but I'm not that tough yet). Then start with 1.5 liters from the portal (but not from Portal taps (since there are water problems up there right now), fill up in Lone Pine before heading to the Portal) and fill up as needed along the way. PLENTY of water on the trail this year.
You'd normally want around 3 liters per person from Trail Camp to the summit and back. (again, everyone is different)
There's a spring around switchback 23 (although it's flowing over almost a dozen switchbacks right now) that is the last water before the summit. It's flowing VERY well right now so you cannot miss it unless you take the wrong path to the summit
3. I've been hiking with my favorite Keen hiking boots, they are unfortunately not water proof. Should I take along water shoes for portions of the trail where we will encounter water? I hate to change my hiking boots 2 weeks before go date.
Outpost camp is the only place that people have said they still see a lot of water in the mornings (the level goes down in the afternoons). Since you'll be going through there early, you may want to consider water shoes. You could leave the water shoes on the other side of Bighorn Meadow as long as you remember to pick them up on the way back.
4. We will be keeping a close eye on weather. Should we have a turn around time or a cut-off? For whatever reason we are delayed or go slow, should we have a time we turn around. Say T-storms are expected at 1pm, it takes 3 hours from summit to tree cover, if we arent at the summit by 10am, turn around to avoid running into bad weather? Or am I over thinking this?
Yes, for a day hike a good rule is to have a turn around time that you will always turn around if you haven't reached your goal and stick to it. Lightning or not.
You don't want to be under a tall tree in a thunderstorm. Or at the summit.
As for how to gauge the weather when going towards the summit, if thunderstorms are forecast and there are dark clouds around the summit when you're at Trail Camp, best not to continue from there.
If you're past Trial Crest and dark clouds start coming in and you see any signs of lightning, turn back immediately.