I’m surprised that no one ever answered your question, and i only started coming to Apple Community recently, sorry for the late notice and late reply, and hope this is still helpful to you 🙂
Here are the answers to your questions:
– Try not to deplete the battery as this actually harms the battery lifespan.
– charge your MacBook Pro when it has about 5-10% battery remaining, MacBook and MacBook Pro has a power management system integrated since Mac OS 10.11 El Captain, if your battery is below 10%, Mac will automatically reduce hardware performance to preserve remaining battery charge and you will start experience lag, functioning delay, longer loading / processing time. it doesn’t hurt the battery itself, but the lag could drive you mad.
– Keep your Mac on the charge all the time while you use it would damage the battery? Theoretically NO.
The general technology has developed so much so as the battery technology.
Theoretically what Today’s laptop would and should do is to deactivate the charging when it calibrated the charge in the battery. which means there is no more electric charge goes into the battery anymore.
From personal experience, i’m doing that all the time, i only take it off the charger when i actually take my MacBook outdoor. And my battery does exactly the same performance and duration as the first day i got it (owned my MacBook Pro for 1 and half year now)
And i never power off my MacBook Pro since Apple has done so well with Mac’s power management system (way better than Windows and by miles)
But, if you aren’t using the MacBook, and put it into sleep mode or shut down. then disconnect the charger. What i said above only applies to MacBook being 100% charged when you are using your MacBook or it’s powered on all the time.
To be honest, the excuse of Overcharge laptop by leaving the charger connected is simply not acceptable. because this is a basic precaution that portable electronic manufacturers should have been dealt with long time ago
