You are correct, when using the two-weapon fighting combat option, the Shield Master feat means any attacks you make with a shield are made at no penalty. Since all of your attacks are made with a shield, none of them are penalized. The Two-Weapon Fighting feat is irrelevant, since all that does is reduce those penalties, while Shield Master eliminates them.

However, Shield Master also requires the Two-Weapon Fighting feat. That means you cannot use Shield Master without the Two-Weapon Fighting feat,¹ as feats’ requirements must be met in order to use them. So no, it doesn’t get you out of taking that feat.

1. _{Unless you’re somehow getting it as a bonus feat that doesn’t need to meet requirements; I’m not seeing anything in your question that suggests you are, though.}

You are correct, when using the two-weapon fighting combat option, the Shield Master feat means any attacks you make with a shield are made at no penalty. Since all of your attacks are made with a shield, none of them are penalized. The Two-Weapon Fighting feat is irrelevant, since all that does is reduce those penalties, while Shield Master eliminates them.

Note that, normally, Shield Master also requires the Two-Weapon Fighting feat. That means you cannot use Shield Master without the Two-Weapon Fighting feat, as feats’ requirements must be met in order to use them.

But if you use a slayer talent to get a ranger combat style, and choose from the “weapon and shield” combat style in order to get Shield Master without meeting its prerequisites, then yes, you get to skip the Two-Weapon Fighting feat, and it won’t matter since all it does is reduce the penalties that you’re ignoring. Note that this approach does prevent you from taking Improved Two-Weapon Fighting and Greater Two-Weapon Fighting, which could be painful. You’ll have to think long and hard about whether or not it’s worth it to burn two or three feats on one or two extra attacks at low attack bonuses.