Depends on the branch requirements, some require a long term lease, some a certificate of residence, some a yellow book and some a combination of the above. Ask at your local branch for their requirements.
No you don’t need two certificates from Immigration, Chonburi DLT accept one original and one photocopy when applying for both licenses. Immigration will tell you that you need two but you don’t. Save yourself 300 baht and just get one.
PIT = Personal Income Tax. Your UK income is subject to PIT and paid to the UK government via the PAYE system. PIT is one of many forms of withholding tax charged at source.