Can I legally leave my household without getting into legal trouble if I am 16?

No, you cannot leave the house at 16 without getting in trouble. When you are 17 you can petition to be emancipated to live apart from your parents.

Can I purchase a firearm now, specifically a shotgun or any long gun?

Your conviction will have no impact on purchasing a firearm. However, if you are still using marijuana that would prohibit you from buying a firearm.

Can you have a superseding indictment which lowers the charge after the charge has been dismissed?

Unless the indictment was dismissed with prejudice you can be re-indicted.

I got a speeding ticket, no License, failure to identify, and no Insurance tickets in Texas 15 years ago, they want 1500$?

You should have taken care of it 15 years ago. No statute of limitations on pending charges.

Probably means the terms of his probation were modified to send him to SAFP.

this was previously answered. Get an attorney to help you locally.

Maybe, maybe not, best thing to do is get an attorney if a revocation has been filed.

You should talk to a local attorney about the situation.

Should not stop you from you becoming a phlebotomist.

Will i be drug tested at court?

Some judges drug test at the time of sentencing someone and without knowing what judge it is hard to say. Best to stop using drugs.