"Last Monday (it) was one week since we left for our trip." or "As of last Monday, we have(or had) been on our trip for one week". I think here you mean that from last Monday, 7 days before that you left on your trip. If this is correct, then I think the correct version is " As of last Monday, we had been on our trip for one week." "(By) tomorrow it will be(have been) one month since we got divorced"? For this one I would say "Tomorrow it will be one month since we got divorced." The "By" can be in there but you would use it if you were stressing that there's only one day to complete a month since the divorce. For example, if you needed to turn in some documents within one month of the divorce and you're emphasizing that those documents are due tomorrow. I'm not sure if you can use the "will have been" here.

We have been on our trip for one week since last Monday. Tomorrow will be one month since we got divorced