5-7reps is BEST for mass building hands down! If you do a little research you will find that this is the optimal rep range for building mass. This will trigger your fast twitch to increase gains. You say you stopped doing that rep range because you had terrible form. Well guess what. that means you drop some weight until you can do 5-7 with GOOD form! It is too easy to be able to push up more then you should by throwing other muscles into the exercise. Practice good form and do the amount of weight that you can with good form for 5-7 reps…
Congratulations on your choice to transform your body.
Yes, I think dual workouts would do more harm than good. The only people getting away with this are steroid-users because steroids speed-up the recovery process. As a natural, your body needs more rest time.
I would leave the cardio if your goal right now is to put on some muscle. Pick one goal and go for it. If you want to gain muscle, you don’t need to be doing cardio.
If you’re trying to accomplish both goals (building muscle and losing fat) at the same time, you’ll either accomplish neither or (if you’ve great genetics) get there very slowly.
While it is possible for some people to build muscle and lose fat simultaneously with the diet I recommend; it’s preferable to pick goal and focus solely on that.
Hope that helps,