Hi Jennifer,

I'm not experienced with injecting trees. I would really call a tree service and talk to a licensed arborist.

https://hyg.ipm.illinois.edu/article.php?id=833

I believe Fall is a good time to inject for iron. 

As far as the insecticide goes, I'm not sure what problems  you are having ? I'm not a fan of using them unless you have an extreme problem.

https://xerces.org/systemic-insecticides

 I would plant another tree not too far away that is more suited to your soil conditions, preferably a native that won't need special care and actually want to grow there. That way you have an established tree growing when your other two may be in decline.

https://www.dnr.state.mn.us/treecare/best-native-yard-trees.html

Thanks for your question,

Sally Granath

Hennepin County MG