For any mitigation solution, you have to ask 3 questions 1) How much does it cost 2) How much space does it require 3) How does it scale

Planting trees performs excellent on (1) but terribly on (2) and (3). According to Bill Gates' recent book on the topic (where he aims as best has he can to summarize the state of knowledge as per today), you'd basically have to make a big forest of the rest of the world just to offset US emissions.

In addition, it's not enough to plant trees, you have to plant trees where there wouldn't have grown any if you were not planting (i.e. making forest out of non-forest). This is going to put an additional huge strain on the need for land to feed more mouths, which is a problem that is only becoming more precarious.

Yes and trees only store CO2 while they are alive. Once they die and rot, it's released back into the environment. Also monocultures with fast groing trees that ideally can be harvested come with their own problems. Planting trees is good way to do something now, but it's not going to solve the problem forever.

You're supposed to turn the Sahara into a green forest. The obvious problem is that you would need the cooperation of those countries.