Regarding the questions of memory charms and whether dementors have the ability to make victims of the Memory Charm relive their memories:
First let us distinguish memory magic:
The Memory Charm, with the incantation of "Obliviate". Used by Gilderoy Lockhart on Harry and Ron, Barty Crouch Sr. on Bertha Jorkins, Hermione on Rowle and Dolohov are just three of the examples I will be discussing later.I have always had the thought, like some other readers, that the Memory Charm does not literally erase a memory, but covers it up with....blankness. It's more like using liquid paper rather than an eraser.
A False Memory Charm is to alter the memory instead of erasing it, which is the one Hermione used on her parents, and possibly similar to the magic Voldemort uses on Harry, to make him believe that Sirius was being held captive at the Department of Mysteries. Slughorn also used this one on himself.
Torture can also affect the memory. The Longbottoms' memory have been twisted and distorted not by memory charms, but by immense torture. In this case, it seem like a very hard eraser has been used to rub their memory off. Torturing people can also break memory charms, as Voldemort has done to Bertha Jorkins. This can be seen as the very hard eraser rubbing off the liquid paper, and to some extent, tearing the whole paper, thus making Bertha Jorkins losing her mind like the Longbottoms before she was being killed. It is unknown why Rowle and Dolohov have not remembered seeing Ron in the cafe after being tortured by Voldemort, but my guess is that Voldemort was just so emotionally angry that he did not use the Cruciatus Curse specifically to break the memory charm, but merely to punish his servants. Unlike the above two methods of playing with memories, this method cannot choose a time span of memory to be forgotten or remembered.
The confundus charm also seems eligible to be placed in memory-related magic, but I still do not understand why it's possible to use this instead of the Imperius curse, with similar results (Snape confunding Mundungus with the 7 Potter Plan).
Legilimency also contains memories, but memories in this case is only part of the pack. Legilimency is used to read minds (whatever Snape puts it), and thus gives the oppresor a chance to navigate into the part of the brain which stores memories. I see legilimency as a higher level of memory-related magic, but it doesn't limit itself to memory.
With dementors let's imagine the cases of 3: Obliviated, false memory implanted, and tortured. Dementors suck out all the happiness, and so, rather like osmosis, bad memories reveal themselves from the subconscious. It takes the willpower to hold on to the happiest memory, to produce a Patronus.
I do not believe that a victim of the Memory Charm can relive any bad memory that has been erased. Lockhart will not remember anything that has happened before he accidentally obliviated himself. If he comes across a dementor, he might remember of loosing his memories, but nothing before that. (His case is quite complicated because I do not think he intended to make Harry forget everything. I choose to believe that he only wanted to cause Harry to forget about things related to their adventure, but that the backfiring of the charm altered its original purpose).
False memories are easier to be uplifted, and I believe that dementors can indeed make victims of this charm relive bad memories which have, in any case, been altered. Say for example, a person's devastating memory of seeing someone die has been altered to seeing a person sleeping. Should this person come across a dementor, I'd say that he will remember, suddenly, that he saw death instead of sleep. Hermione's parents, after believing themselves as Wendell and Monica Wilkins, for instance, might suddenly remember that their daughter fell down her bicycle when she was young. Whether they would cling on to this memory of having a daughter, or see it as a hallucination, I do not know.
Tortured people will definitely see themselves tortured, or in any case, any worse memory that happened after the torture, should they come face to face with a dementor. Like the victims of total Memory Charms, I do not believe that they will remember anything that happened before.
Apologies for making this so long. Please tell me if I skipped a logic.