I imagine dualling the rest of it would probably be the most sensible option in terms of preventing this sort of thing happening again. I'm not sure how realistic or expensive that would be in engineering terms but given how much this has cost I'm sure they will want to mitigate the risk in future. Dualling it doesn't prevent a ship blocking it again (that's actually probably fairly likely given the ever increasing size of ships) but means if one side got blocked they could use the other side in the interim. The only other option I can see to prevent something like this would be to significantly widen the entire canal so that a ship couldn't get wedged in this way. But you'd be talking more than doubling the width in some sections. Probably just as expensive as building another parallel section.
Anchors, anchor chain and all the associated equipment (windlasses etc) on most ships are only designed to hold the ship in relatively calm conditions and are not designed to take excessive loads. Case in point, there are currently many cruise ships anchored off the south coast of the UK as the industry is temporarily shut down. A lot of them have had anchor related incidents (lost anchors, snapped chains, damage to windlasses etc) in bad weather over the winter as they are using the anchors outwith the parameters they are designed for. The Marine Accident Investigation Branch have actually just released a bulletin about that this morning.