1. You can if you want to be lazy about it. The truth is you want to use as many different alt-text phrases as possible. Each alt text is an opportunity to target another keyword. To use the same alt-text over and over results in spamming that phrase. Not a good idea.
2. Do people normally search google for that product code, I bet not. I would name the picture with a keyword or phrase. For example, if you sold roses use "Long-stem-red-roses" for a picture of a red rose.
Also note I used hypens and not "underscores".... "long_stem_red_roses". Underscores just don't come across properly.
3. Again you can be lazy and use the same for both, the title and alt-text. Understand you are giving up an opportunity to target an additional keyword.