Linda is very clear about Willy's problem: She has witnessed the ghosts of the past showing up in his mind, and she is quite aware of his problems with dementia and delusions, and she knows that he had attempted suicide.
However, Linda is a much more complex character than her lines: She is a submissive 1940's 1950s wife whose only duty was to obey her husband and nurture the family. At this point, however, we know that Linda refers to Wily "as a little boat in search of a harbor." THAT harbor is Biff: Linda feels that the problem with Willy is NOT the dementia, but the loss of his hopes for Biff (and, secondarily, for Happy)
This is why she is mad at her children- Because they let themselves go and failed to comply with Willy's wishes. Willy lived vicariously through his two boys, and saw Biff as a man who would do great things. Yet, neither Happy nor Biff had much to show for, nor much to live for.
The rubber hose incident was that basically Linda found it, and knew right off the bat that Willy intended to die of carbon monoxide poisoning. He had tried to kill himself before anyways to cash on his life insurance so Biff could move on. Why did she put it back? Because it has been obvious trough the play that Linda would never go against anything Willy does and, secondly, because she still felt like problems would fix themselves once her children got themselves together. She was an endless dreamer, often disappointed, but never surrendering.