They were early enough into the dive that they would have abandoned the dive after losing contact, they would have only been about halfway to the bottom, and they still would need communication to even find the titanic. So if they lost contact, the mission would have been aborted inside the sub, because they wouldn't have any hope of completing the mission without the aid of the surface vessel.
There are several emergency surfacing options available on the sub in addition to the primary propulsion method. If the sub lost contact, it would have surfaced. That is what happened in several other missions this company has made where communication was lost.
So the fact that communication was lost and no attempt to surface was made, suggests that there was a catastrophic failure. At the pressure they were at, the smallest leak would lead to immediate implosion. The most likely scenario is that they experienced a crack or defect in the hull that lead to implosion. It was likely over before anyone onboard even was aware of a problem (or within seconds of it).
Not if they are doing something useful to others.
Are hikers who get lost on a mission?
(1) a specific task with which a person or a group is charged
(2) a definite military, *naval*, or aerospace task
You should seriously find better things to do with your life than obtuse rebellion against dictionary definitions. It makes you look like an illiterate nonce.
If they are on the surface, getting them by sonar is not possible, but aerial surveillance might find them. However, things are very hard to see in the ocean if there are waves. They might have an AIS locator which would transmit their position and an emergency signal, but maybe they skimped on that too, because the submarine-rated ones are expensive.
Volume is also an issue, the Titan is far smaller than a military sub, so the sonar spike corresponding to the implosion will be shorter and of a smaller amplitude.
The sonar on their surface vessel should have picked up the noise, if they have one that does recordings and passive mode. But I guess they were too cheap for that as well, since no regular sonar surveilance of the sub is mentioned in the press.
Time isn't on their side.
Official reports have been saying 40 hours a few hours ago.
It has come out that it is wildly untested and optimistic estimate. The longest mission they had conducted onboard was 12 hours. So the theoretical limit may not be realistic. If there was panic onboard, that would increase oxygen usage dramatically, which could cut as much as 25% off the estimate (which is nearly a day in this case).
In other words... it is grim.