It depends on how closely the anime followed the manga. If it was a very faithful adaptation, you might want to start from the next chapter. But sometimes, there could be differences, so it's best to check online manga guides or discussion boards for precise recommendations.
Generally, no. Once the sale is completed, the manga is yours. However, some stores might have specific return policies if the item is damaged or unopened.
After episode 1000, it's best to start with the chapter that directly continues the story. You can find this by checking the official manga release or referring to manga reading platforms that provide the correct sequence.
I'd suggest starting from the chapter that corresponds to episode 1060. You can check online manga forums or dedicated manga sites for the exact chapter number.
Sure. When you've mastered Japanese through the manga method, Yotsuba is accessible. It might have some challenging parts, but your new skills should help you get through it.
Their relationship could turn into a bitter rivalry again. Maybe they both regret the one - night stand immediately. Hermione might think Severus took advantage of her in a moment of weakness, and Severus might think Hermione was just being a silly Gryffindor. This would cause them to start arguing and competing again, but this time with an added layer of personal animosity from the night that they shared.