It really depends on the purpose of the net in your grow. If attempting a SCROG, you'll introduce it early so you can use it as a plant training tool. If you are just using it as a trellis support and just allowing the plant to grow through it, it can be added later on, though it's still recommended to get it up before things get too bushy.
Managing the back half of your tent can be tricky. I'm lucky my tent(Vivosun 4×4) opens up almost all the way around for easy access to all sides. All you can do is the best with what you have.
Defoliation is a contentious topic amongst the grow community, you're gonna get all kinds of answers on that. Some live by hard rules and perform it at set grow weeks, some do it more by "feels" and judge when their canopies are getting a bit overcrowded to do it, and others will hold onto and cherish every last leaf lol. The "correct" answer really comes down to your environment, the particulars of the strain you're growing, and possibly other training methods you're already doing. I'm a "feels" guy, I don't always defol but do when things get congested. You're more likely going to need to defol if you crowd your tent with several topped plants. Plants growing in their more natural (untopped) state don't really need defols as much unless you're doing SOG(sea of green).
Good Luck!