If you want a better mental model of how snapshots work think of it like this: every file on disk is comprised of data blocks. The filesystem keeps a map of what data blocks belong to what files and how many references there are to these blocks. If a data block is referenced in the current running version of the filesystem as well as snapshots B C and D there will be 4 references in that block's count. If B C and D are removed the reference count drops to 1. If the file is then modified (ie: overwritten with new data) the number of references to the original block drops to zero and it will be marked to be freed for reuse.

How long data is kept around depends on whether or not anything on the filesystem (files or files contained in snapshots) still points to that data. It's a lot like reference counting and garbage collecting in memory if that helps. All of this to say: it doesn't matter if you delete snapshots from the middle of a sequence, later snapshots will retain references to data that they need so the data will be kept until the reference count for it drops to 0.