Can I send my Ontario closing funds by Interac e-Transfer instead of a bank wire?

Generally no, at least not for the full amount. Interac e-Transfers carry daily and per-transaction limits set by your bank that are far below what a typical real estate deposit or closing fund requirement involves, so e-transfer is not a practical way to move the kind of sums involved in an Ontario home purchase.

Real estate deposits and closing funds are instead typically sent by bank wire or delivered as a certified bank draft, both of which can handle large sums in a single transaction and are the methods your lawyer's office will normally expect and be set up to receive. If you are unsure what your specific limits are or what method your lawyer requires, check both your bank's transfer limits and your lawyer's office preferences well before your funds are due, since discovering a limit problem on the day money is supposed to move is a completely avoidable way to put your closing at risk. Confirming this early also gives you time to arrange a wire or draft properly rather than scrambling at the last moment.