Probably you are right there (I can’t fully understand it). Nevertheless, if we take a look back at HM case (Henry Molaison), the 2/3 of the HC/EC/ complex was removed (https://en.wikipedia.org/wiki/Henry_Molaison).
Being a bit naïve here, my take is that EC and grid cells are mostly involved in new episodic memory formation. HM kept the capacity to form procedural memory. I think that hippocampal replays use the grid cells to determine where in the cortex the replay should be done (and where the sensory stream should be fast-stored in the HC). Therefore, is just a translation mechanism from HC addresses to cortex addresses (some sort of TLB 
) You might observe grid alike behavior elsewhere because the “TLB” can have multiple levels (?)
I don’t think that HM lost the capacity of using abstract ideas. Only the ability to create new episodic memories. Replays are needed because up in the hierarchy the events are really sparse in time. Note that replays even occur when we are awake.
Just my 2 cents
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vpuente