Yes. You can go back and explore to find all the collectibles.
Yes, you can. There are various side quests and expansions that you can undertake even after completing the main story.
Yes. There are side quests and other content.
In my opinion, starting with the main quests in Velen like 'The Bloody Baron' is a great start. It sets the tone and introduces important characters. Then move on to some of the related side quests in the area that expand on the story and characters you've met. After that, head to Skellige and follow the main quests there which often have a more epic and mythical feel.
Yes. Skellige related content is still available. You can go back to Skellige and do all the things you could before, like exploring the villages and fighting sea monsters.
One good strategy is to follow the critical path of the main quests in the order they are presented. This will keep you on track with the overarching story. After that, you can go back and do the side quests in each area you've visited. For example, after finishing the main quests in Velen, do all the side quests there before moving on to the next area.
No. While many heroes can be gained from the story in King's Raid, there are also some heroes that are obtained through other means, such as special events or gacha systems. For example, limited - time event heroes are often not part of the story - based acquisition.
Well, there's the theme of destiny. Geralt's search for Ciri is tied to the idea of fate and what is meant to be. Also, the theme of love and relationships is prominent, with his romances with different characters. And the theme of prejudice exists too, as Witchers are often shunned by normal people.