To “circulate” (circulate) means to move from one place to another, or to pass from one person to another so that the information is shared (everyone gets to see the information).

To say that the memo circulated last week means it was sent to many people at the office last week.

Since everyone has seen the memo, Chris says that he sees no reason to cover his proposals in depth at this time.

He says he sees no reason, meaning it is not necessary for him to cover, or to discuss, his proposals in depth.

To cover something “in depth” (depth) means to discuss it with a lot of detail.

You probably read the newspaper very quickly, but if you see an article (a story) that interests you, you may read it in depth (read the entire thing very closely).

Chris says that he doesn't want to cover his proposals in depth at this point in time.

The expression “at this point in time” means now, or right now, at this moment.

At this point in time, for example, we are about one-third of the way through the vocabulary explanations for the formal meeting; that's where we are right now.

So, Chris is saying right now, I don't want to talk about the memo in depth because people have, or should have, already read the memo.

Chris says that we do, however, need to open a dialogue.

To “open a dialogue” (dialogue - sometimes spelled dialog) is to begin to talk about something, or to start a discussion.

For example, in recent years, countries have begun to open a dialogue about global climate change.

Chris wants to open a dialogue, or start a conversation, about which changes the company is going to implement.