"Did the nursery sell me diseased trees?"

There is no way to know that for sure. The spores of various plant fungal diseases can exist independently from the plant so no way to know if the trees were infected before or after purchase.

Did the trees come with a warranty? Many nurseries will guarantee their plants (usually for whatever reason) for a set period of time, often a year.

How was the problem diagnosed? You will need an accurate ID first but many fruit tree fungal problems can be addressed by fungicidal sprays but the proper timing of these sprays is often critical.

Okay, this is going to sound like a stupid question, but please bear with me. We are spraying the apple trees with Neem to prevent insect and fungus problems. So my husband mixed up a lot of Neem in the sprayer. Now we need to spray the copper fungicide on the pear trees. Can we just empty the Neem out into a bucket. Thoroughly rinse the sprayer, and put the copper fungicide in the sprayer. We only have one sprayer. I don't want to mix two different chemicals and have a problem! We are new to this, and I just don't know. Thank you!