This question has been asked numerous times by F-1 students so a complete answer will be helpful to many. Thank you for asking.
1. Is my plan going to affect my H1b petition filed by my full-time job employer? For example, will the USCIS challenge my maintenance of legal status because I'm also working for my own company that I established as side business?
No. The petition will not be affected. Your OPT-EAD does not carry a rule that prohibits this.
2. Will my full-time job employer know that I have my side business? Do I need to report this to my employer to comply with any legal requirements?
You are employed on the basis of an EAD. Your reporting obligation is to regulatory agencies that govern taxation and labor and not to your employer unless there is a conflict of interest. For example you can't get into a printer business without permission if your employer does the same business.
3. Do I need to report my new business to my school for them to report to SEVIS? Note that I will always keep my regular full-time job and I know I need to report that to the school. But not sure about my own business.
Change of employment must be reported. You are not changing employment here.
4. Will my own company be eligible to hire OPT/CPT students for work?
I would advice against this. You have to be an e-verify employer and this brings lot of regulatory attention. You don't need to fear this but gets you into lot of red tape.
5. If my status changes to H1b, at what point of time do I need to transfer the ownership of my company to someone else?
Immediately. Can you transfer ownership right now to someone you trust? It will be much easier for everything.