I would advise a separate post. That would make it easier to discuss, since replying to replies can get messy.
Why are clubs always accused of refusing to release players for the international windows?
Clubs do release players for these matches. They don't always like it, and prefer top players who don't have international commitments, but the players do get released.
Clubs are often not prepared to release players for matches or training sessions scheduled outside the international window. Why should they?
There are side agreements, like the one which sees French clubs willing to voluntarily release Argentinian players for training camps ahead of the Rugby Championship. The quid pro quo is that Argentina doesn't select these players for matches during the June Tests. The IRB officially frowns on such deals but there's no other way of balancing Northern club commitments with a Southern test schedule.
Te'o has signed to Leinster and England don't select players from overseas leagues. He won't be considered unless he moves to an English club.