Hi,
We can see you have many questions. We will respond to each one below.
A. Yoast schema suffices or do we need a structured data plugin
Yoast outputs a basic set of schema as described here: https://developer.yoast.com/features/schema/plugins/yoast-seo/. The plugin also has a feature which lets you select some additional schema as well. This guide explains more: https://yoast.com/help/selecting-content-types-in-schema-settings/.
Finally, the plugin outputs HowTo and FAQ schema as Gutenberg blocks.
If you are wanting some highly specific schema, you may need to consider an alternate plugin.
B. And will there be some kind of conflict if we are using Schema structured data for WP and AMP for adding a knowledge graph, etc.
If you are using an alternate plugin there is very much likely to be a conflict. To resolve the conflict you will need to cross-reference the schema that each plugin is outputting and disable it. We say this as outputting the same schema many times from different plugins can cause confusion with Google.
You would need to reach out to the Schema structured data for WP and AMP plugin authors to see how easy it is to disable their schema.
With Yoast you would need to create custom code using our filters and APIs here https://developer.yoast.com/features/schema/api/#add-or-remove-graph-pieces to specifically target the right schema to disable.
Finally, in regards to the Knowledge Graph, while we are not sure what other plugins generate (again you may need to disable it), Yoast automatically creates the code for the graph. You would need to go to SEO-Search Appearance-General and fill in the data for that section. Then go to SEO-Social-Accounts and fill in as many of the social fields as you can.
This guide explains more about that feature: https://yoast.com/google-knowledge-graph/.
