Very specific questions. Contact an immigration attorney in TX. This forum is not for specific legal advice, but general information. As a general matter, you are expected to seek the stamp within reasonable time otherwise it would cast doubt as to the validity of your employment offer. They do not have to grant your visa event if USCIS approved.Ask a similar question
Technically, you can get a visa stamp at any time during the period of validity of your H-1B visa. However, as a practical matter, if you don't get a visa stamp and start working on the H-1B visa, the consulate is going to have some very serious questions about whether a valid employment opportunity exists for you if you have not begun working on the H-1B visa.
You do not say in your question whether you would be working for the H-1B employer while on OPT, but even if you were, and while that might give you an argument to use, that you were working for the H-1B employer though on a different status, I personally would never want to stand in front of a consulate officer and try to make that argument. They have too much discretion and spend too little time during your interview to give you much hope of convincing them the jobs are the same. The fact you work for the same employer does not mean you are doing the same job.
At the very least, you are begging for a 221g notice asking for more information (and causing a significant delay). During that time, you would have to wait outside the US, thereby jeopardizing your job in the US.
The best plan is to go soon and get the visa stamp, assuming there are no other complicating factors perhaps related to the fact that your employer did consular processing as opposed to a change of status.Ask a similar question