Can initialization of two particle states, for example, two isolated spins (say one of the spin's state is set to be the opposite of the other) be considered entanglement without any physical mechanism to connect them? Will manipulation of such spins externally (same manipulation on both, say flipping the state) be considered equal to a physical mechanism that couples two particles?
As far as I know, simple correlation between two states isn't enough to form an entangled state.
One of the criteria of entanglement points out that if one can factorize the state vector (for example, |phi>=|phi_1>|phi_2>), then the state (|phi>) isn't entagled. In this case the subsystems |phi_1> and |phi_2> are physically "independent", for one can examine the properties of each factor (|phi_1>, |phi_2>) on its own, isolated, and it wouldn't affect the other factor. (I know the explanation isn't accurate, but it can help in some way to understand the whole thing).
Of course there may still be a correlation between them (for example, |phi_1> is set to downward spin, and |phi_2> is set to upward, so in general state |phi> both spins kind of come together with that structure and evolve accordingly to it), but it has nothing to do with quantum properties or interactions.
There was some argument one time (e.g. in this article: http://cds.cern.ch/record/142461/files/198009299.pdf) that on classical level you can see such correlations everywhere. You can send one sock to one of your friends, and another sock (from the same pair) to another. When one of them looks at the received sock, he would know exactly which colour has the sock sent to another friend. But obviously it is not due to some quantum entanglement.
@Eugene's answer is quite good and correct. More generally, there is no protocol for generating entanglement from unentangled states with only local operations and classical communication (LOCC).
Of course there are ways to entangle the qubits that do not involve the two qubits directly interacting, for example a joint measurement on the state of the two qubits to project them into (for example) a Bell state. But this still requires some non-local interaction or measurement.
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