Just be aware that scheduling a cruise is a little different than tweaking a hotel reservation on land. Cruises are priced differently, different weeks/months, different itineraries, different ships. And prices typically increase the more staterooms are sold. When changing dates you will be subject to "current" pricing at the time you make that change, which may be very different from the original fares.
Insurance doesn't really kick in until after your PIF (payment in full) date. Depending on itinerary, that may be 120 - 90 days prior to embarkation. At that point, you would need insurance to offset any cancellation penalty. Prior to the PIF date, there is no penalty for changes (unless booked concierge), and just be familiar with your insurance to know whether it's portable (to the new date) or if you lose it and have to re-buy a new policy for the new date. (Insurance bought through
DCL isn't actually "purchased" until the PIF date, but it may or may not be the best coverage for your needs.)
As to reaching DCL - yeah, it's been tough to reach either DCL or WDW by phone or the past couple of years. Hopefully that smooths out, no idea when. All I can say is try to plan dates that you can be relatively sure won't change. Or book through a
travel agent who will make the contact for you.
If you test positive
at the port (meaning at embarkation testing), no phone call at that time. You are given the option of a refund, which they take info for immediately, or a credit - which would require contacting for a new cruise date. If you test positive prior to arriving at the port, you would need to contact DCL to cancel/rebook (or have your TA contact).