Question-3: Should I consider or preferred Scala based Spark certification only, because Spark is written using Scala and heard that Spark Scala is faster that PySpark?
Answer: You should select certification based on your programming language skill. If you are from Java/Scala background then go for Scala based Spark certification. And if you know Python programming language than go for Python Spark certification.
With regards to performance: that was the case on older version of Spark where Scala Spark was better performant that PySpark. But in Spark 2.x this is not the case (Because of their Optimizer), whatever programming language you use either Java/Scala/Python/R all are same on performance. Only exception to this is User Defined Function.