Welcome PhunkeyPhish.
a) I would say, many paths have a similar start, but is only the farthest reach which counts.
For example one goes to a shopping mall, and just imagine, on the first floor only silver is found, on the second floor only gold is found, and on the third floor there are only diamonds, but to reach each floor one has to take an escalator or go by stairs, which is common to start with for going any of the three floors. It all depends what one is looking for, and if one is to pay the price of it
In a similar way, many paths may seem fine and similar, but is by great good fortune(kirpa), one reaches the highest goal.
b) As for what I have seen in the teachings of Bhagat Jan, Guru Sahiban and Gurmukhs, the main and essential difference between sikhee and other paths, is that in Sikhee, all and everything revolves around the Guru, without whom, nothing can be acheived.
Then also, there is no need of variety of different spiritual entities which do exist, and for us as sikhs they are very much respected, but not at all sought, for our goal and destiny is only: Ik Oankar Satnam.
That is it.
For example, in Gurbani it says:
Bin Satgur kinnay na paayo, bin Satgur kinnay paaya.
This means, without Satguru nobody ever obtained anything(Waheguru Akal Purukh), and without the Satguru nobody ever obtained Him(Waheguru Akal Purukh).
Sikhee means, a path of discipleship, so naturally if there is disciple, there has to be a Guru, otherwise it is not sikhee, it can be anything but not sikhee as per teachings in the Gurbani.
In other paths howsoever respectable, everything depends on one´s efforts or actions, in Sikhee, one has to surrender oneself to the Guru and depend alone on His grace, for the inner ptah is so narrow and dark, that in reference to that Guru, the second Patshahee Sree Guru Angad Dev Maharaj says:
ਜੇ ਸਉ ਚੰਦਾ ਉਗਵਹਿ ਸੂਰਜ ਚੜਹਿ ਹਜਾਰ ॥
Jae So Chandhaa Ougavehi Sooraj Charrehi Hajaar ||
If a hundred moons were to rise, and a thousand suns appeared,
ਏਤੇ ਚਾਨਣ ਹੋਦਿਆਂ ਗੁਰ ਬਿਨੁ ਘੋਰ ਅੰਧਾਰ ॥੨॥
Eaethae Chaanan Hodhiaaan Gur Bin Ghor Andhhaar ||2||
Even with such light, there would still be pitch darkness without the Guru. ||2||
He says, that if 100 moons were to arise and shine in he skies, and in addition there were 1000 suns also, yet inspite of that refulgence and brightness, there would be still utter darkness.
*Moreover Guru Nanak also emphasizes on the supreme importance and the need of such a Guru, as seen in the following 2 verses.
ਨਾਨਕ ਗੁਰੂ ਨ ਚੇਤਨੀ ਮਨਿ ਆਪਣੈ ਸੁਚੇਤ ॥
Naanak Guroo N Chaethanee Man Aapanai Suchaeth ||
O Nanak, those who do not think of the Guru, and who think of themselves as clever,
ਛੁਟੇ ਤਿਲ ਬੂਆੜ ਜਿਉ ਸੁੰਞੇ ਅੰਦਰਿ ਖੇਤ ॥
Shhuttae Thil Booaarr Jio Sunnjae Andhar Khaeth ||
Shall be left abandoned in the field, like the scattered sesame
So you see, it is only by being in the company of Satguru, or call it Shabad Guru, can one reach that abode of eternity and changless Truth of Satnam.
At other place the Bani tells us: Nanak, Naam jahaaz hae, jinee cahreeya se utareeya paar.
With this it is telling us, o Nanak, the Nam is the vessel or aircraft, in which whosoever boards on is carried across.
This Naam refered in the Bani is the Shabad Guru also, for by holding on to the pallav of this Guru, one is ferried across, it does not matter how many thousand other practices one may incurr into, but if one has not got hold of Naam, one is left behind in the painful cycle of births and deaths.
If one goes through the Gurbani with attention, it shows nothing but instructions to love and follow diligently and faithfully the teachings of Guru Sahiban.
Just to summarize in a way Sikhee, it could well be:
Jin Har japeeya se Har hoeeya.
He/She whosoever repeated His holy Name and manifested it within, becomes one with Him without any distinction.
I would suggest you to go to the depths of the utterly sweet and beautiful, uncomparable and unfathoamble Gurbani, and by His grace reach the highest goal.
Stay blessed.
Sat Sree Akal.