Can you purchase medical/dental insurance after you know you will use it?

The Manulife link you provided says:

Your coverage is guaranteed with no medical questions when you apply and pay your first premium within 90 days of your employee benefits ending.

That implies that for new coverage, you will be asked medical questions.And of course your answers will affect whether they accept you, what your rates will be, and what will be covered.

You can submit an application for insurance, but you must disclose all of your existing conditions and your family history. Anything that is relevant must be disclosed on the application form. If you neglect to mention your dentist's quote for the substantial work you need, this is known as insurance fraud (which is a criminal act).

As others have mentioned, it is common for insurers to have a waiting period (also known as a stand-down period) of around 6-12 months after your application is approved before you can make a claim. So that means even if they accept you knowing that you have a pre-existing condition, you could be waiting for a long time before your insurer would cover the claim. They would also typically charge you higher premiums as they know you'll be more likely than average to make claims in the future.

If you decide to wait out the 6-12 month waiting period, your dental health problems could get much worse over that time, or more painful. So most people would just pay for the dental treatment themselves rather than wait that long. There are finance companies that offer medical loans for people who urgently need health or dental treatment but don't have the savings to pay for it. So that is another option for you. But it is cheaper to borrow off family, if you can.

No, all dental insurance that I know of will have waiting periods for major work. Many will not have waiting periods for routine cleanings, but that won't help you.

Aetna has a dental discount program without a waiting period. But, if you shop around, you can probably get work done for the same cost without the discount.