Veer Jee,
I feel Kalki and Nihkalank are 2 different entities.
Nihkalank is the GURMAT'S prophecy about an Avatar(sikh) who will fight evil kalyugi forces.
Kalki is the Visnhu's 24th Avatar and is a prophecy of Hindu matt who will also fight evil kalyugi forces.
Kalki Avatar is mentioned in Kalki Purana and Vishnu Purana.
Guru Sahib in Sri Dasam Granth Sahib has taken the story from puranas and have re-written it. Kalki after winning the world gets arroagant and declares himself God. As a result he is not able to escape Kaal.
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www.sridasam.org]
ਜਗ ਜੀਤਿਓ ਜਬ ਸਰਬ ॥ ਤਬ ਬਾਢਿਓ ਅਤਿ ਗਰਬ ॥ ਦਿਯ ਕਾਲ ਪੁਰਖ ਬਿਸਾਰ ॥ ਇਹ ਭਾਂਤ ਕੀਨ ਬਿਚਾਰ ॥੫੮੩॥
जग जीतिओ जब सरब ॥ तब बाढिओ अति गरब ॥ दिय काल पुरख बिसार ॥ इह भांत कीन बिचार ॥५८३॥
When he conquered the whole world, his pride was extremely increased; he also forgot the unmanifested Brahmin and said this;583.
ਬਿਨ ਮੋਹਿ ਦੂਸ੍ਰ ਨ ਔਰ ॥ ਅਸ ਮਾਨਿਯੋ ਸਭ ਠਉਰ ॥ ਜਗੁ ਜੀਤ ਕੀਨ ਗੁਲਾਮ ॥ ਆਪਨ ਜਪਾਯੋ ਨਾਮ ॥੫੮੪॥
बिन मोहि दूस्र न और ॥ अस मानियो सभ ठउर ॥ जगु जीत कीन गुलाम ॥ आपन जपायो नाम ॥५८४॥
There is no second except me and the same is accepted at all the place; I have conquered the whole world and made it my slave and have caused everyone to repeat my name.584.
ਜਗ ਐਸ ਰੀਤ ਚਲਾਇ ॥ ਸਿਰ ਅਤ੍ਰ ਪਤ੍ਰ ਫਿਰਾਇ ॥ ਸਭ ਲੋਗ ਆਪਨ ਮਾਨ ॥ ਤਰ ਆਂਖ ਅਉਰ ਨ ਆਨ ॥੫੮੫॥
जग ऐस रीत चलाइ ॥ सिर अत्र पत्र फिराइ ॥ सभ लोग आपन मान ॥ तर आंख अउर न आन ॥५८५॥
I have given life again to the traditional and have swung the canopy over my head; all the people consider me as their own and none other comes to their sight.585.
ਨਹਿ ਕਾਲ ਪੁਰਖ ਜਪੰਤ ॥ ਨਹਿ ਦੇਵ ਜਾਪ ਭਣੰਤ ॥ ਤਬ ਕਾਲ ਦੇਵ ਰਿਸਾਇ ॥ ਇਕ ਅਉਰ ਪੁਰਖ ਬਨਾਇ ॥੫੮੬॥
नहि काल पुरख जपंत ॥ नहि देव जाप भणंत ॥ तब काल देव रिसाइ ॥ इक अउर पुरख बनाइ ॥५८६॥
No one the repeats the name of the Lord-God or the name of any other God goddess;" seeing this the Unamanifested Brahma created another purusha.586.
ਰੱਚਿ ਅਸ ਮਹਿਦੀ ਮੀਰ ॥ ਰਿਸਵੰਤ ਹਾਠ ਹਮੀਰ ॥ ਤਿਹ ਤਉਨ ਕੋ ਬਧੁ ਕੀਨ ॥ ਪੁਨ ਆਪ ਮੋ ਕਿਯ ਲੀਨ ॥੫੮੭॥
्रचि अस महिदी मीर ॥ रिसवंत हाठ हमीर ॥ तिह तउन को बधु कीन ॥ पुन आप मो किय लीन ॥५८७॥
Mehdi Mir was created, who was very angry and persistent one; he killed the Kalki incarnation within himself again.587.
ਜਗ ਜੀਤ ਆਪਨ ਕੀਨ ॥ ਸਭ ਅੰਤ ਕਾਲ ਅਧੀਨ ॥ ਇਹ ਭਾਂਤ ਪੂਰ ਸੁਧਾਰ ॥ ਭਏ ਚੌਬਿਸੇ ਅਵਤਾਰ ॥੫੮੮॥
जग जीत आपन कीन ॥ सभ अंत काल अधीन ॥ इह भांत पूर सुधार ॥ भए चौबिसे अवतार ॥५८८॥
Those who conquered, the made it there possession they are all under the control of KAL (death) in the end; in this way, with complete improvement the description of twenty-fourth incarnation is completed.588.
Kalki Avatar in Sri Dasam Granth is NOT a prophecy, its just a story re-written from Puranas.
Nihkalank is a Prophecy mentioned in Bhattan de Swaiyai -- in praise of Satguru Ramdass jee Maharaj.
ਸਤਿਗੁਰਬਚਨਕਮਾਵਣੇਸਚਾਏਹੁਵੀਚਾਰੁ॥