I recommend that you complete all of problems for one day before moving on to the next day's activities. Sticking to one concept at time before moving on to the next concept will help promote mastery of each topic. It's also important to note that there are several math topics that build on previous math topics, so it's a good idea to build a strong foundation before moving on.

Given all of this, don't worry if you fall behind. Day 42 is the last day that you learn new content, and the remaining days are devoted to taking practice tests and working on weaknesses. So, you can always catch up then.

Regarding your second question, I think it's a good idea to master one section at a time. Here's the outline of all 60 emails: https://www.beatthegmat.com/mba/gmat-guide-outline

Cheers,
Brent