Anyways I wanted to know which of these sentences are correct and what should be the usage for it since I can't find the exact usage on internet.
It depends on your interpretation. For example, this sentence :
research and innovation has always been a fundamental theme of my academic endeavours.
This would just become grammatically correct if you are interpreting the subject singular. Meaning, "research and innovation" only denotes one.However, on the contrary, this one:
research and innovation have always been a fundamental theme of my academic endeavours.
This sentence would only be grammatically correct if you are interpreting the subject plural. That only means that the subject does not pertain as one --research and innovation are separate things.1). research and innovation has been always been a fundamental theme of my academic endeavours.
2). research and innovation have been always been a fundamental theme of my academic endeavours.
Actually, you can place the adverb "always" after or between has and been.Hope it helps:)