can i use a 61W power adapter instead of a 91w adapter

You can. It will charge the battery slower and when using your Mac, the smaller power supply may not be strong enough to charge the battery depending on the system load.

Apple lists all MacBook models in the list of compatible devices with the 96W adapter so you should be fine. I know there are lots of articles and blog posts about charging iPhone with more powerful adapters and how it could or could not impact battery life but I'm not sure about how this would impact MacBook batteries. My understanding was always that the device will only use the power it needs and not that the adapter will always supply what it can output at maximum capacity if that makes sense.

There are no 2019 MacBooks, perhaps you have a MacBook Pro.

Is the reverse also true? Can a 96W adapter safely replace at 61W adapter that is worn out?