Dear Nura,
May peace be with you.
Please see my responses in blue to your questions below.
1.is the highest rank in heaven only attainable by prophet Muhammad? People talk about a rank called al-wasila that can be attainable by only one person. is it true? Can only one person attain the highest rank in heaven?
There is no support for this assertion in the Quran. The Quran simply presents 3 broad categories. 'wakuntum azwajan thalathatan' (And you will become three kinds) (56:7)
The exalted people, those FOREMOST in the race (sabiqun) 56:10, (2) people of the right hand (56-8) – righteous and (3) those of the left hand (56:9) - unrighteous.
Arguably, there will be numerous shades of ranks to accommodate the diversity of endeavours amongst humankind but no such exclusivity is mentioned in the Quran.
Even those believers (men and women) that obeyed God and the Prophet joined the noble company of the prophets, truthful, martyrs / witnesses and righteous (4:69).
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"And for all there will be ranks from what they do, that He may fully compensate them for their deeds; and they will not be wronged"
2. If the highest rank in heaven can be attainable by many, is it only attainable by prophets?
Please see my response to question 1 above which argues for no exclusivity. Along with belief, it will be one's deeds that will ultimately determine one's rank.
3. Can normal men and women attain the highest rank in heaven and can be brought nearest to Allah? can a normal human like me strive for such a lofty goal? Or is it futile or non-sensical for me to strive for it?
From a Quran's perspective, yes. Again please see my responses to the first two questions above. You can indeed strive for such a goal as can anyone else God willing.
4. I have read that, we are in this world to try and come nearest to Allah, is that possible for normal humans? I feel deep in my heart that it is possible if we work really hard.
Yes you are correct, it will all depend on the extent of our endeavours.
I hope that helps, God willing.
Joseph.