Turning into a one way street, one notices part way thru street that large construction vehicles have blocked the road. Is it legal to turn around and go the wrong way on a one way street to avoid traffic blockage? What other legal remedies remain for homeowners on street and other traffic caught in such a situation? Is this situation similar to say a downed tree?
The proper process for the contractors is to block off the street with a "Road Closed - No Through Traffic" sign before the obstruction is in place, which implicitly allows people with no way out of the temporarily blocked one way street to get in and out.
In practice, someone prudently getting their car out of the blocked street, as in your example of a downed tree, by having hazard lights on and moving slowing and with great attention to traffic in the direction that the street is going, would not be ticketed. These things are a reality of urban living and police don't punish people for this quite ordinary behavior under the circumstances by citing them for it unless the stop is really a pretext to really stop someone for a reason that they don't have a legal basis to do so for.
If there was an accident while someone was going contrary to the one way direction to get out causing personal injury or property damage, then making a "negligence per se" argument (i.e. that someone is liable in negligence as a matter of law because they violated a law intended to precent the accident in question) stick in a traffic case based upon a technical violation of the one way street regulation in this context, would also probably not hold water, when a jury assessed the negligence of everyone involved in the accident,
The person backing out may have a technical traffic sign obedience violation, the car hitting that car would have a "failure to keep a proper lookout" and "failure to keep a safe following distance" violation, and the construction crew would probably also have liability for failure to follow proper procedure to close the street that they were blocking. A jury would have to decide who, if anyone, to hold responsible for the injuries in an accident caused in this situation.