Yes. Where there is dark brown sugar used, you may use light brown sugar instead. Brown sugar adds a slightly caramel-like taste, due to the molasses content; this gives the dark brown sugar the deeper colour and richer flavour. 

Yes. I have made this recipe refined sugar-free, and low-sugar for family and friends. To make this refined sugar-free, in the batter where there is 4 tablespoons of brown sugar used, you can use a diabetic-friendly brown sugar alternative, or any sugar alternative made with allulose that won't spike your blood sugar (like this), or you can use 3 to 5 tablespoons of pure maple syrup.

For the streusel, I would use a granulated brown sugar alternative to give it a similar consistency. Some branded brown sugar alternatives are more bitter in flavour than others, and all are not created equal. To balance the bitterness, you may or may not need to add 1 tablespoon of maple syrup to your streusel. Here are some helpful brands that I like and have used in the past. View it here. 

Baked goods are generally best consumed the same day they are made, or within 3 days. This cake doesn't usually last too long in our house, but if you have leftovers use these tips for storing:

Immediate Storing: After the cake has cooled completely, wrap the cake with cling-film, baking parchment, or aluminum foil. If you use baking parchment, you can also store it in a cake tin or cake container with a lid closure. Then, store it on the kitchen counter at room temperature for up to 3 days, or place the cake in the refrigerator, if preferred. When you're ready to enjoy it, heat in a microwavable plate in 30 second intervals, or warm the cake in an air fryer for 3 to 4 minutes at 350° F/180° C.

Freezing: Wrap individual slices in a freezer-safe bag or container and store in a ziptop bag (this will keep well up to 3 months). When you are ready to enjoy it, thaw it out on the counter for 1 hour beforehand. Then, heat in a microwavable plate in 30 second intervals, or warm the cake in an air fryer for 3 to 4 minutes at 350° F/180° C.

Everyone's oven varies! If your oven cooks a bit high and it has already been 30 minutes and you're worried about the edges of the cake, here's what you can do.

To prevent your cake from overbaking or getting too crisp, just carefully remove the cake from the oven, and immediately place a sheet of foil loosely over the top of the cake pan (you can crimp the sides but do not enclose it tightly). This will help prevent your cake from getting over baked. Place it back in the oven and bake for the remaining range of time, or until the cake is done.

But, please remember, this is a coffee cake, and it should have a deep golden brown colour, and have a slight crisp edge, which means it is done. Make sure to check the middle of the cake with a wooden skewer or toothpick, if it comes out clean, you're good to go!