Answers hoping from those who have done Leh trip more than ones...
Here are some question need to be answered about Leh area:
1.) Is the shimla - spiti - kaza - manali - Leh route open in the last week of May like last 2 day to 1st week of June. Say May 28th onwards...?
No, Kunzum opens up first week of June or even later.
2.) Between Shimla and Manali via spiti valley, are there quality fuel pumps and not selling loose?
There are fuel pumps at Narkanda, Rampur, Jeori, Powari. Quality..hmm..cant say
3.) Is shimla - kaza - manali route tougher or worst than Manali - Leh route?
No, post kaza is tougher till gramphoo. Elsewise Manali - leh is tougher
4.) How cold it is on the spiti side?
Very!
5.) Are visitors allowed in the Kye monastery?
Yes
6.) Can Leh - Tso Pangong - Leh be done in one day?
Easily
7.) Can Leh - Nubra valley - Leh be done in one day?
Yes, but then there is no point going to nubra if you are not going to stay there. you wont be able to visit all the places.
8.) Which areas are the point where your bike looses a bit of torque becoz of less oxygen and less combustion... I need to be careful here as I have a pillion along?
Baralacha La, Tanglang La and Chang La.
9.)During this time (last week of May and 1st week of June) does it rains on those routes? (Shimla - kaza - manali - leh - nubra - kargil - srinagar)
No, though it can snow.
10.) Does karizma goes good on that terrain with a pillion along?
Fine. But Karizma is notorious for choking at high altitudes.
Let me hear about these 10 questions first...
Shoot more 
Regards,
OM