Using the syringe to inject leaves, will the leaves be wounded,...

This is where experimental skill comes in. It took me and several other members of the lab, as well as a large number of collaborators and additional colleagues many, many attempts to get it right. The thing to get right is not to wound the leaf, which can happen with "heavy hands" or mishandling. It is very common with students and first-timers. Over time, a good experimenter will learn the amount of pressure that is required to correctly infiltrate the leaf tissue. Applying water 30 minutes below on the leaf surface that you will be infiltrating has helped as well. It enables the liquid to be dispersed more easily into the apoplast. What is important to keep in mind is NOT to force the liquid in, as this will damage the leaf surface. With practise and correct positioning of the syringe, you will slowly learn that with the right gentle finger pressure, the liquid actually fills very passively and easily into the tissue.