1) The concepts he understands a 'light' and 'darkness' are muddled in this very bright very chaotic river. On passive scans his scans almost want to skip over it as too confusing, too much to deal with.

2) The sensory overload will be emotion based- every possible emotion you can think of slamming into him all at once, threatening to drown him.

3) Assuming these are similar to dark corridors- in that they work at all because he can use the to transport himself around in a world as well as inter-world, opening one will result in feedback strong enough to knock him back about five feet. The portal will then snap shut, he wont be able to use them.