Yes, you have the right to assist your wife, unless that assistance rises to an "unreasonable" level. An unreasonable level example would be claiming you need to assist your wife in a communal shower at the same time the junior high school girls volleyball team is showering. It would be unreasonable because showers are not emergencies and an alternative time could be arranged.
As for signage you request, not required by law. As a practical matter, the world would be one great big sign if we had to put up signs for every situation that could possibly arise.
Finally, the ADA only covers the relationship between business and the disabled. The public can be as crude, inconsiderate, nasty, appalling and downright stupid as they want.
