Most definitely cut or tear up your leaf as much as possible, this will really help with the extraction of the medicinal compounds from the leaves. You can certainly strain what you have and do another warm extraction with cut/torn leaves in the oil you already infused.
This one is a bit tricky, since you want to retain some of those volatile compounds and it is best to use a warm infusion for a fresh herb. However with the warm infusion many of the volatile compounds can be released. On that point, unless your slow cooker can be turned down very low, I actually find slow cookers to be a little bit too hot/warm unless you put water in the slow cooker (just about an inch) and a tea towel or cloth under the jar to buffer that direct heat. Then make sure you have some muslin or cloth over the jar.
Another option is to do a ‘cold’ infusion, with the jar in a brown paper bag in a sunny/warm window. Still cut or tear the Lemon Balm up first. Then let it macerate for 6-8 days and strain. After straining it, let it sit on a counter for 12-24 hours and decant the oil off the top (there will likely be water sitting at the bottom from the plant matter).
Wishing you enjoyment and success with this lovely infused oil!