Based on the info I've read throughout the years on testosterone, the symptoms you listed seem fairly indicative of low testosterone. How old are you? How's your energy level? How's your ability to recover after exercise?
The side effects of TRT are pretty minimal. I think at most you'd probably have to monitor your estrogen level with labs that your doctor should hopefully set up and agree that it's a wise idea for you. A TRT dose is around 50-100mg per week if I recall, so if you chose to stop at some point, you could transition back to being "natural" pretty easily. Depending on your age, you may need some Clomid and more labs to monitor your natural test and estrogen level but again, it's nothing to worry about. (I should preface all this by saying this transition back to natural may be a bit "tougher" or take longer if you're older as the body just doesn't "bounce back" like someone who's younger.)
I don't think trying to increase T naturally is a "long" process. I think it's an ineffective process. Lots of products out there have wild claims and junk science behind them so I'm wondering what your plan is do increase yours naturally? Longjack, trib, avena and zinc immediately come to mind but I don't have much faith that they'll improve the symptoms you listed.
1. Injection is the fastest way to get T into your bloodstream, I believe. Transdermal applications have to cross the skin and is poorly absorbed. There are others on this board who have more knowledge on this for sure.
2. A TRT dose is conservative but I think you'd feel a bit more energetic and a little more hornier. Depending on the ester your doc uses, if you feel anything it could be from one day (if it's suspension but I don't think docs use that) to a week (if it's enanthate or cyp but I don't think they use those either).
3. No. Absolutely not.
4. No. Absolutely not.
You won't incur side effects or harm anything, but you won't feel like Superman and notice a significant improvement off one shot, either. If anything, having the one shot in your system will help calm some of these fears/concerns you have.
When it comes to using T, there seems to be two kinds of people: those who immediately see the benefits of being on T/TRT and are excited by those benefits, then others who are averse to T where the risks loom front and center in their mind. This is where educating yourself about T will help and having a critical mind regarding the sensationalism out their about their risks is important. Right now as I read your post, you seem to have a fair amount of worry and concern and that's fine, but that leads me to think that no matter how much T helps you (if it does help with your symptoms), it's always going to bother you or worry you that you're on it, and because of that you won't get as positive of an experience as you could, if that makes any sense.
I'm 42. I've been on T in the past for cycles here and there. And as I age, part of me looks forward to being on it forever for better mood, energy and quality of life. But along with being on comes the need (for some) regular labs, HCG shots, procuring these things, possibly the use of Clomid/Tamoxifen, and all that can turn into a headache over time. I had concerns initially but I tried to educate myself about the risks and benefits and such. If you're going to try it then go on T with an open mind and a good attitude. Maybe keep a journal so you can monitor how you feel. Stay in close contact with your doctor if that helps you feel better about taking it.