Can the defaulting owner go to court to stop or delay a power of sale I'm trying to buy?

Yes, this is a real possibility, and it is one of the reasons power of sale purchases carry more uncertainty than an ordinary transaction right up until closing. A defaulting owner generally retains a right, up until the sale is actually completed, to pay out the arrears and reinstate the mortgage - sometimes called a right of redemption - which can stop the sale from proceeding even after a buyer has agreed to purchase. An owner can also go to court to challenge the process itself, commonly by arguing that the lender did not follow the required notice or other procedural steps.

For a prospective buyer, this means an agreement to purchase a power of sale property is not necessarily as certain as an ordinary Agreement of Purchase and Sale until the transaction actually closes, and a deal can fall through for reasons entirely outside your control. It is worth discussing with your lawyer how the specific purchase agreement addresses this risk, including what happens to your deposit if the sale does not proceed.

Understand going in that a power of sale deal carries a real risk of falling through before closing, and confirm your deposit is properly protected if that happens.