You ask very relevant and valid questions - I'm always surprise that candidates in Malaysia, even senior ones, don't necessarily understand the way so many things can go wrong during an offer process.

1. I've seen offers retracted enough times due to medical test results that had red flags in them about a candidate, to know that your offer is never 100% secure until you've passed ALL mandatory pre-hiring processes and checks, AND you have gotten the offer letter in black and white (even an email does not suffice as that's not a legal document the way a letter of employment is).

So forget about backdating your letter. What you actually need to do is just politely inform HR that you'd like to wait for medical results to be out and for you to sign your offer letter before tendering your resignation. This is after all, what cautious, reasonable, and smart candidates with good judgment and understanding of risks do.

We are talking about just a few days difference, which isn't going to move the needle on how well you perform at the next company. If HR isn't on your side, try reasoning with the hiring manager. What will he do if he were in your shoes? Will he dare resign without passing a medical check - what happens if he discovers cancer in that process? Is the job offer still valid?

2. Yes you can. Once again, pay attention to the fact that there's always a tiny disclaimer in every offer letter in Malaysia stating something along the lines of 'employment or offer is valid subject to satisfactory passing of all pre-employment checks, etc.'

Another advice for when you finally resign: your current boss may freak out, refuse to meet you, try to stall for time to get a counter offer approved, whatever. Assuming you are dead set on leaving, remember to date your letter of resignation and keep HR cc-ed throughout. Your resignation is effective the day you sent that email/letter in, not the day HR or your boss accepts your resignation.

That's fine, always tough to strike a fine balance between not pissing off your ex-employer (from whom you probably would need to contact again sometime in the future for references) and wanting to demonstrate your enthusiasm about the new role to your new employer.

What you are doing - informing your boss before an official letter - is fair and smart.

In response to his text about keeping HR out of it, you can gently affirm again that you are 100% committed to ensuring that a smooth and responsible handover, but that you will be emailing him your official letter of resignation shortly (keeping HR out for now). And in exchange for keeping HR out of things (for now at least), your resignation takes effect from the date indicated in this letter and email. What matters here is that the letter of resignation has the correct date in it (which better be soon now that you've committed to a start date and you have your letter of offer already).

Why a scanned letter of resignation embedded in an email is your perfect solution is that no one can tear it up in dramatic fashion, and it is already dated for when you need to ultimately forward it to HR for them to issue an acceptance of resignation and start calculating your leave balance / final pay.

Bosses freaking out and feeling betrayed or wanting to guilt you into staying is all part of the fun of resigning I guess. Again, never worth it to piss anyone off. Reassure him of your handover plan and that you are open to be contacted after leaving in case anyone has any questions so he can start making the necessary arrangements. You never want to burn that bridge, but you won't be intimidated or bullied either into thinking you can't leave if he doesn't acknowledge reality.