You can just turn around and re enter immediately. Although be aware some land entries require minimum 1 or 2 days between exit and re entry. By air exit in the morning and re enter in the afternoon.
once you automatically become Thai tax resident it means you are subject to Thai tax law. The requirement to pay Income tax will be dependent on your personal financial circumstances. You may or may not have a tax liability.
you automatically become Thai tax resident after residing 180 days or more in a tax year inside the Kingdom. That does not mean you have to file taxes it just means you are subject to Thai tax law.
If you are resident in Thailand for 180 days in a tax year you are automatically Thai tax resident and subject to Thai tax law. You may or may not be subject to tax filing. That all depends on your personal situations. You may never have to file for tax or you may have to file from day one. It is your unique financial circumstances that will determine if you need to get a TIN and file taxes.
perhaps you need to clarify what exactly you mean by Widows pension. Are you talking about a private pension beneficiary or the UK state War Widows pension. More info gives a better answer.