Hello Raymond, I'll answer you second question first because the explanation is a bit lengthier.
There has been a lot of debate over the years about how accurate the information in Search Console is, particularly some of the information in Search Analytics. Personally, I found the information to be inaccurate at worst and misleading at best. As an example, I searched through the number of impressions one of my client's small local sites had received over a period of thirty days and discovered my client was ranking for some long tail keywords that were misspelled and had somehow received hundreds of impressions from those keywords. I then checked the volume of those keywords in Adwords and saw immediately that there was a discrepancy between what Adwords reported (an extremely low yearly volume) and Search Console (extremely high monthly impressions).
So either A) impressions means what we traditionally know it to mean - someone typed in a keyword and landed in the SERP and saw the listing. Which would mean the information is inaccurate (possibly tracking bot traffic?). Or B) impressions means something else entirely in Search Console, which means it's misleading.
This was a roundabout way of saying don't take Search Console data as gospel. There isn't a clear cut guide on how the information is gathered and disseminated, meaning you cannot quality check it for accuracy the way you can with Google Analytics.
To answer your first question: adding a secondary keyword that's relevant is unlikely to negatively impact you. I would find it highly suspect that Google would rank you lower for adding MORE content to your website, and the additional keyword could improve CTR, which has been associated with higher rankings for awhile now, as a sort of positive signal with a short half-life. Just make sure it makes sense for your users and the topics are related. "Tree removal and tree trimming" is acceptable, whereas "Tree removal and ice cream" would be confusing and I would be unsurprised to see a negative impact from that.