You can't get a tourist visa in Thailand so what you're being told is correct. Your best bet right now is to get the 60 day covid unable to travel extension and then work something out to stay longer.
That's the minimum salary requirements for someone from Singapore. For the UK, US, Australia, Europe it's 50k per month. It goes down to 25k for people from Vietnam, Cambodia, Laos, and Burma.
just to add on to my last message, most countries will have primary taxing rights to income earned in that country. The fact you don't live there won't change this. The purpose of the double tax treaty is to prevent the same income being taxed twice, it doesn't mean all of your income will be taxed in your country of tax residence.
For example, I live and work in Thailand. If I had any UK income, say rental income, then this will be taxed in the UK as it is UK sourced income. The fact I'm not living in the UK is irrelevant and the double tax treaty won't stop the UK government from taxing it, it will however stop the Thai government from taxing it.
not necessarily, if it's Canadian source income then Canada may still have primary taxing rights which is why you'd need to check the tax position in Canada.
a TIN doesn't prove you're a tax resident in Thailand. I have a UTR in the UK but I'm not a tax resident in the UK.
Whilst the number of days in Thailand plays a part in helping to determine tax residency you will also need to take into account whatever the residency and tax rules are in Canada.
if they can't get a visa to continue their stay then they'd have to leave just like they've always had to, that's not going to change. Remote working as in working from where you want is a long long way off from happening for a whole host of reasons.