Sansanii सनसनी

I was looking for a word that could be translated as "sensationally" and the word I've come across is sansanii. Can the word "sansanii" be used with a negative conotation?

"Sansani" सनसनी is most of the times used in this negative connotations; so you can go ahead and use it.

driver / chaalak रिक्शाचालक

In a BBC Hindi article I found this word: रिक्शाचालक rikshaachaalak. Is this word used in colloquial Hindi speech?

Rikshachaalak (रिक्शाचालक) means driver of Rickshaw and is a very commonly used word in Hindi. It is not of new or recent usage but a pretty old one. You can speak it and will be understood by Hindi speaking population. 

Parii परी Vs Jinn जिन्न

In Urdu, we use Parii परी and Jinn जिन्न to mean the same thing. Is it same in Hindi also?

In Hindi Parii परी and Jinn जिन्न are not the same. Parii परी is considered essentially female while Jinn जिन्न can be both, may be, but mostly used for males. As such in Hinduism, souls don't have any gender but when souls are stuck without taking rebirth, hence called Atma आत्मा, Jinn जिन्न or Pret, or things like that, a gender is used to describe such souls. Yet in truest form a soul doesn't have any particular gender in Hinduism.

"rabri" or "raabri" in Hindi

Is it "rabri" or "raabri" in Hindi? Is it spoken also as "raabri" in all parts of India?

It is spoken both as Rabri रबड़ी and Raabri राबड़ी even in Hindi. 

I have heard it everywhere; some people speak रबड़ी and some speak राबड़ी and both are correct.

आप लोग हाज़िर हुए Vs आप लोग हाज़िर थे

आप लोग हाज़िर हुए Vs आप लोग हाज़िर थे : Any difference and which is better usage?

आप लोग हाज़िर थे (aap log hazir the) indicates towards a statuesque meaning, "you people were available". 

आप लोग हाज़िर हुए  (aap log hazir hue) indicates immediate state/status, or an event of appearing, meaning "you people became available"

Fursat me Vs Fursat se (फुर्सत में Vs फुर्सत से)

Can you explain difference in usage of फुर्सत में Vs फुर्सत से, if any?

"fursat me" फुर्सत में and "fursat se" फुर्सत से are used almost interchangeably, though at times fursat me is better and at times fursat se is better. 

"Fursat me" फुर्सत में indicates towards a "static" nature in which the person is at leisure, apparently for a long time.

"Fursat se" फुर्सत से indicates immediate attention towards the "present" time/status where the person is at leisure at present, may or may not have been like this for a long time. 

Adjective same for males and females

Can you give example of same adjectives used "same" for males as well as females. 

सुन्दर राजा, सुन्दर रानी

मंदबुद्धि लड़का, मंदबुद्धि लड़की 

साफ कपडा, साफ कंघी

Spirit of the law

What is the equivalent term for "Spirit of the law" in Hindi?

कानून की भावना

No Entry. Please use the other door.

How do you translate this in Hindi: "No Entry. Please use the other door."

In Hindi, we can write it as:

प्रवेश वर्जित. कृपया दूसरे दरवाजे का उपयोग करें.

Jhanjhanahat झनझनाहट

What does Jhanjhanahat (झनझनाहट) mean in Hindi?

Jhanjhanahat झनझनाहट is the word to describe something which is a sensation or sounds "jhan jhan". For example, when someone slaps one on the ears, one can say that "मेरा कान झनझना गया |"

Or see application else here: कहीं आपके हाथों में झनझनाहट तो नहीं

Or here: पैरों में झनझनाहट या एनीमिया जैसे लक्षण