Most people want better rights for Workers, yes, Marxism just provides a sound base for analyzing why Worker's rights aren't good in the first place. It's a helpful lense of analysis.
You don't need to be a Marxist to want better rights for Workers, not at all, but Marxism definitely helps. That's why leftists are almost exclusively Marxists in some fashion, even Anarchists acknowledge Marxian analysis of Capitalism.
Yep, for sure! You don't even have to think he was 100% correct about literally everything to gain value from Materialist analysis and viewing conditions through a Marxian lense.