I would recommend that you investigate this, as the issue could blow up to be a bigger mess, especially with that large amount of money involved. Yes, you should check with Council to see if any DA was applied for or granted. You need to review the by-laws to see if a renovation by-law was registered (though the work seems to have been done when strata law was different, so there may not be one.)
You have been told the sunroom was an unauthorised renovation, but there is another way of managing that, as well as the Owners Corporation demanding that the common property be restored to its original condition (ie, without the sunroom). You can retrospectively apply for a renovation by-law to be approved that authorises that structure, and the fact that it’s been in place for over 20 years, without the OC complaining or objecting would imply that it would be approved.
It’s good that the OC is treating the repairs as being carried out on common property, and paid for by the OC. But if it turns out that a previous owner did unauthorised work that contributed to the current problem, then you, as the ongoing owner, could be liable for some of the cost of its remediation, as well as contributing to the levy costs. It might pay to get a strata lawyer (or an independent strata manager) involved to look through this, to ensure that you don’t become liable for a big bill.
And yes, a million dollar remediation cost will probably be paid for by all the owners, by raising a special levy. What is in your current Capital Works and Administration funds? Has your OC been planning this work for a long time, and has raised the necessary money, or is this unplanned? Note that the special levy can be raised in one hit, or spread over a longer period, but work cannot start until sufficient money is in these trust funds to pay for each stage of the work. Strata loans are also a possibility. You should get involved in your OC, for your own benefit.