So much to tell you! Here goes:
For starters, congratulations on making the choice to better your health. That's often the hardest part of getting into training, so you've made a huge step already.
Don't rule out abdominal/lower body definition. It's not an impossibility to see that over time and with proper nutrition and training.
To answer your questions:
Yes, it absolutely makes sense. In fact, that's going to happen no matter what you do. The more muscle mass you have, the higher your resting metabolism will be. As you continue your training, assuming you're doing the right type of training (more on that in the next answer), you will be gaining lean muscle, which will help in losing body fat. That said, for fat loss, diet is arguably the BIGGEST part. I know you mentioned not being able to necessarily follow a strict diet, but getting SOME kind of guidelines would help very much.
That's completely up to you. If you mean steady state cardio (treadmill, elliptical, etc), I would say "no". Steady state cardio does next to nothing when compared to proper interval training using resistance. The majority of weight lost with steady state cardio is water weight, which will be gained back as soon as you drink water. The thought process behind doing cardio for fat loss is that it keeps you in the so called "fat burning zone". While you do "cardio", the primary fuel source is your fat stores, but that doesn't mean you're burning the most fat possible per minute. That's where you'd be better off with interval training. By definition, anything that increases your heart rate is cardiovascular exercise. Your heart, muscles, and lungs will get stronger with interval training and you will be burning fat in the process.
You will know by about week 4 of any given program if it's right for you. That's when you will notice physical change, assuming you've stuck to the program strictly. I'd love to tell you there's an "easy button" and that you can loosely follow a program and still see results quickly, but that's not (typically) the case. It takes hard work to see serious results, but if you're willing to put at least 3 hours a week (split amongst 3 sessions) into your fitness, and you bust your a** during those sessions, you'll be golden. As far as "how long should you give it..." it depends on the type of program. The programs I create are periodized. As you progress, the program progresses. It will bring you through cycles of building lean muscle, building strength, and increasing power output. All of which will increase your muscle mass and decrease fat stores. Typically, I write 8 week programs that are set up to give you optimal results by week 8.
I'd be happy to answer any other questions you may have. Feel free to shoot me an email and I'll get back to you as quickly as I can: [email protected]
Also, if you'd like to check out what I have to offer, my website is www.imperiumathletics.net . Through January, I'm running a 50% off promo using coupon code: getfit16 . I know my programming will get you to where you want to be, so hopefully we can work together to get you there. I look forward to hearing from you soon!
-Coach Chris