1. They didn't "see" the devastation at Hiroshima until August 8th, when the team of scientists and observers they sent verified what had happened and sent a report back. The attack itself damaged communication networks and the Japanese high command understandably wanted a true technical assessment before assuming that US statements about what had happened were true.

2. There is evidence that the Japanese high command and Emperor were less impacted by Hiroshima than the Soviet declaration of war against them on August 8th/9th. It is hard to disentangle these things, but this is why they met to discuss their possible surrendering on August 9th, not Hiroshima, and not Nagasaki (which they got news of during the meeting).

3. There is no strong evidence that Nagasaki changed their opinion one way or another. It may have reinforced existing opinions, but it was not a radical shift.

4. They did not agree to unconditional surrender even after Hiroshima, the Soviet invasion, or Nagasaki. They agreed to a conditional surrender that exempt the Emperor from full Allied authority. This was what was communicated on August 10th, and the US rejected it. It was not until after further (conventional) bombing, and an attempted coup, that the Japanese Emperor himself announced a full, unconditional surrender on August 14th.

5. As noted, it is a point of scholarly contention and disagreement how much "weight" to assign the atomic bomb(s) and the Soviet invasion in tilting the balance in the Japanese high command towards surrender. It is clear both had an impact; there are various opinions on whether one could imagine surrender happening without one or the other effect.

For more on this sort of thing, Hasegawa's Racing the Enemy is very good, even if you don't end up agreeing with his final conclusions (that the Soviet invasion was more important than the atomic bombs).