Hi,
I don't understand first part "As it is on a subdomain blog with an add to cart feature, would interlink building be helpful in exchanging link juice."
link juice has nothing to do with PPC long tail keyword.
For second part
Yes long tail keyword gives high conversion rate but search volumes would be very limited so I would suggest you to use broad match modifier to target long tail keyword.
e.g +summer + dresses
Hope this helps.
Thanks