Just to clarify are you wanting to use the wireless charger and the wall charger at the same time?
You can use it however you will keep it hot, which might affect its lifespan. You can drain the battery lower, but you'll start using up more of your battery's power cycles when you do. Leaving your smartphone on its wireless charging pad won't destroy the battery though.
Yes it should be safe to charge the portable charger and wireless charge your phone at the same time. You can even connect a USB type c as well and charge a third item