For the record, here's what the manuscript variants look like.

I think it's pretty easy to see why it could have been altered to the plural in the first case; and in, say, John 6:28 the disciples ask how they can "work the works of God" -- so it's clear that some sort of collective "working" could have been envisioned here ("we must work...").

The second instance might be slightly more complicated. While the plural definitely seems to be the more difficult reading, I think we have to pinpoint why this is so. On one hand, it's pretty easy to understand how it could have been changed to first-person, as similar "...sent me" texts can be found throughout John: 5:37; 6:44; 8:16, 18; 12:49, etc. But on the other hand, it may be precisely because of how rare "sent us" would be that might lead us to consider the second "us" simply as a secondary assimilation to the first plural.

But since "We must work the works of him who sent me" seems perfectly sensible, I think we should slightly prefer the plural in the second, too, following P66, P75, etc.

The question about the implications for the "voice" of the (Johannine) community bleeding through here is very interesting. And if we did adopt the plural in both cases, then John 9:3-5 would read as follows:

3 Jesus answered, "Neither this man nor his parents sinned; he was born blind so that God's works might be revealed in him. 4 We must work the works of him who sent us while it is day; night is coming when no one can work. 5 As long as I am in the world, I am the light of the world."

The oscillation between first person person plural and singular here would be slightly curious, to be sure. Also, in 6:29 -- which directly follows the disciples' question about how they can "work the works of God," as I mentioned above -- we read

Jesus answered them, "This is the work of God, that you believe in him whom he has sent."

But you know, this oscillation reminds me of other instances in John, e.g. where we have an oscillation between first and third person. In John 10, we find

35 If those to whom the word of God came were called 'gods'--and the scripture cannot be annulled-- 36 can you say that the one whom the Father has sanctified and sent into the world is blaspheming because I said, 'I am God's Son'?

(Here we have yet another occurrence of "sending" here.)

But I think if we really want to talk about some of the more transparent instances of the voice of the Johannine community (and/or the redactor) bleeding through into the text in this way, then we should look toward the beginning of John 17 -- yet another text that involves "sending":

1 After Jesus had spoken these words, he looked up to heaven and said, "Father, the hour has come; glorify your Son so that the Son may glorify you, 2 since you have given him authority over all people, to give eternal life to all whom you have given him. 3 And this is eternal life, that they may know you, the only true God, and Jesus Christ whom you have sent.

I think there's a much better case to be made that we have secondary redaction here.