Need your guidance for express entry

[sivasatish007]

Hi,
Following is my profile.

Age - 27
Masters in US
B.Tech in India
IELTS : L 8.0 R 7.0 W 7.0 S 8.0
Experience - 5 years pro; 1.5 as Grad Assistant during my Masters

Questions
Can I do my ECA just for my masters ? Is there anyone here who did that ?
My stamping expires in August but my 797 is valid through April 2018. Can I travel to Canada between Aug 2017 and Apr 2018 with out stamping ?

Thank You.
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[all_the_best]

My stamping expires in August but my 797 is valid through April 2018. Can I travel to Canada between Aug 2017 and Apr 2018 with out stamping ?

You can travel based upon approved I 94, but I think will need stamping while coming back.