Don't know about your part of the world, but in mine it is illegal to do electrical work on a house you don't own AND live in. And the questions you asked make it pretty evident you don't have any business doing what you're doing. Of something happens you will be liable for any injuries and...
Everybody's an electrician. The NEC allows handle tied breakers under certain circumstances, and if you follow the rules there shouldn't be any issues.
On a MWBC handle tied breakers are fine. The code requirement is only that the disconnect (in this case the breakers) has to turn off all...