Hi @joel.dalby, welcome to the forum! To answer the different questions you had:
No, you can not “unmark” streets that you have completed, short of deleting the activity from CityStrides entirely. That being said, many Striders would still go back to complete St Mary’s Gardens, for that honest-to-yourself sense of completion 
Regarding the distance to complete a node @dallas.devries is right, that any node within the distance from your GPS (not your run necessarily, but where your GPS data points to!) will be marked complete. Some other comments about node completion appear here.
90% of nodes need to be completed for the street to count, although Jim (CityStrides’ creator) has discussed the idea of a hard mode where 100% would be required.
This is a known issue with how the data is brought in. This thread has a long conversation about it: Double Streets
If that is the case for you, you can add the erroneous city to this tracker, where it will eventually be cleaned up.
In general you can search the forum for these kinds of questions, as many people have had similar questions before. This is an awesome site with an active community, welcome to!