An immediate resignation letter ends your employment without serving notice. Be brief and factual, state the reason only if you are comfortable doing so, and acknowledge the notice clause — many contracts allow you to pay in lieu of notice, and offering that keeps the exit clean.
Whatever your employment contract specifies — commonly two weeks to three months. If the contract is silent, statutory minimums in your country apply. Check the contract before you name a last working day in the letter.
You are not required to. 'Personal reasons' or 'a new opportunity' is enough. Detailed complaints belong in an exit interview, not in a permanent file document.
Usually yes, and it creates a timestamp, but attach the signed letter as a PDF. Many HR systems require the formal letter for the personnel file regardless of how it was sent.
No. Resignation is your decision. An employer can hold you to your contractual notice period, but they cannot refuse to accept that you are leaving.