From the Deathwatch book, it looks like Parry is based entirely on your own skill, not the opponent.

If you have a WS of 55, and your opponent has a WS of 55 and rolls a 01 for attack (hit, 5 degrees of success) and you roll a 55 (Challenging +0 WS Test), do you negate the hit? The parry rules (242) appear to support that reading.

It would also follow that your chance of successfully parrying an opponent would be the same whether their WS was 5 or 95; it doesn't get any harder to parry an opponent based on their WS since the roll is based only on your Weapon Skill.

Is this correct?

Yes, that's correct.

Correct.

Thematically; parrying is akin to slightly pushing or knocking the attack away from your body, so the parry test is to see if you can hit the enemy weapon to do this.