Yes, you can. There are various side quests and expansions that you can undertake even after completing the main story.
Definitely. After the main story concludes, there's still a lot to do. You might come across hidden quests, new areas to discover, and opportunities to further develop your character and skills.
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. You can go back and explore to find all the collectibles.
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.
Yes. There are side quests and other content.
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.
It varies, but on average, if you focus mainly on the main quests and don't do too much side content, it'll take you roughly 55 hours to finish the main story of The Witcher 3. However, if you like to take your time and explore every nook and cranny, it could stretch to 70+ hours.
The main story of The Witcher 3 is quite lengthy and can take dozens of hours to complete, depending on your playstyle and how much you explore.
The main plotlines in Witcher 3 story include Geralt's search for his adopted daughter, Ciri. He travels across the vast and diverse world, facing numerous monsters and enemies along the way. Another important aspect is the political turmoil in the different kingdoms, with various factions vying for power.