For the P90X questions - I did about 60 days of it before life got in the way. It's not going to increase your muscle volume, it will increase endurance however. P90X focus is, as you stated, getting more lean muscle tissue and dropping fat. I didn't notice much of a cardio increase while doing it. There are dumbell routines, but it's focused more on reps than weight. I will say my core strength increased a ton on P90X, though I think that was due to the yoga (see below).
I'm starting Insanity on Sunday. The focus there is mainly cardio, with some attention paid to strength training (again, more repetition focused than max weight focused).
As for core, have you considered mixing in yoga on your off days? I know it's "gay", but I found it to be extremely helpful personally. You can probably find a yoga routine on one of your cable/satellite on demand channels (when I had Time Warner, there were like 20 different free yoga routines if you knew where to look).
you cant do both at the same time, youre either going to lose fat or gain muscle. in the start you can lose fat while keeping the same weight because youre new again, but that will taper off.
pushups, dips, pullups, sit-ups, cardio, cardio, cardio, and eat healthy. It does the trick for Rocky and I.
Gour - Nothing is better for your core then your basic compound lifts. Do a barbell routine that includes squats, bench, deadlifts, mil press, curls, bent over rows, etc. As time passes then you throw in the pretty boy isos like dumbbell curls, chest flies, things like that.
In what order and on what days (assuming starting on Monday) would you suggest someone just getting back into the gym do these exercises? Right now I'm doing upper body (focus on chest and triceps) on the first day (say Monday), and lower on Tuesday with rest on Wednesday. Then repeat Thurs, Friday, with dropsets on one or both. Rest Saturday, then cardio Sunday..
Currently I do my RDL first followed by inclined bench on Mondays/Thursdays and squats after before leg press on the following day. Is that a good approach?
When I started back working out after college, I worked out Mon., Wed., and Friday doing the following routine:
Squats (2 progressive sets, followed by 3 heavy sets)
Bench (2 progressive sets, followed by 3 heavy sets)
Deadlifts (2 progressive sets, followed by 3 heavy sets)
Mil Press (2 progressive sets, followed by 3 heavy sets)
Bent over rows (2 progressive sets, followed by 3 heavy sets)
Curls ( 3 heavy sets)
Calf Raises (3 heavy sets)
Abs
I would do heavy weight on Monday, really trying to push myself (call it 100%). Light on Wed. (call it 60%) and 80% on friday. Wouldn't do deadlifts on Wed. b/c of the strain it places on the lower back, and would do 60% on deadlifts on friday. I also took between 2-3 minutes of rest between each set of squats and deadlifts. I took about 1-2 minutes between everything else. No rest between progressive sets.
Abs you can pretty much do whatever you want to really because the compound lifts utilize the core as well. I did 3 sets of 25 reverse crunches. I did this routine for about 3 months then switched to a mon., tues., thurs., fri. split routine.
Didn't read past the OP, but there is a severe lack of pull-ups in your workout.