Yes you can extend IT in Thailand at an immigration office for an other 30 days it cost you 1900 thb ....it is the best, or you just " overstay 5 days but this cost each day 500 so 2500 thb....
And as described before the list, must have copies of your passport , the departure card and the visa stamp as well....and a proper hotel adress
No! Because your visa b is related to the business who gave you a work permit.if you are out of the job normaly you have to change your visa , because u can only stay 7 days more if the wok permit is revoked or cancels