Acer rubrum

I cannot dismiss early stage gummosis by looking at the picture above. I think a local expert should look at the tree. If this appeared on my roses I would say it is the work of a sawfly but on citrus? In the meantime I would isolate the tree if potted, treat the wounds and the soil with copper and if more of it appeared I would cut off the affected canes making sure the wood exposed looks nice and healthy. To investigate further ones needs to look at the state of the foot below the soil so a little digging may be in order. The fungal disease called 'Citrus gummosis' starts at the root and foot but there are also other types of gummosis. Also take a look at this, paying attention to 'citrus gummosis' and 'Rio Grande gummosis' https://cals.arizona.edu/extension/ornamentalhort/plantprotect/citrusdisease.pdf

Yes. Definitely Sambucus. Geographical location would help with id.

Even new leaves in spring will show typical identifying characteristics. A doubly pinnate leaf will be doubly pinnate however young the leaf. A location and some clearer close up pictures would be helpful. If your location is truly 9b it is unlikely to be 'earpod' tree which is at home in zones 10 - 11. There are many Fabiaceae trees with similar pinnate leaves and I very much doubt a plant app can sort them out without other features such as flowers, fruit etc. and certainly not from these photos.