Actually both KQ6 and Silver Lining would be right because KQ6 is both less than half a year and half a year after KQ5. The intro mentions that Alexander sails for three long months, which means if Cassima later states it's been six months, that Alex's moping in the throne room occurred three months after KQ5. So technically the game does occur less than six months after 5. It just has a three month introduction after it starts.
Ya, I suppose the "less than half a year" and six months would be about same. Just one makes it sound more like less than six months (5 months or less), the other one sounds more specific.
Basically from in-game dates in KQ6, we learn that Cassima has been safe in her room for "six months" (it's not a direct reference to being "six months" after KQ5 however, just the period of time she spent in seclusion), Jollo mentions that after Cassima's return directly after KQ5, she learned of her parent's death, but there is an undisclosed period of time that she spent with Jollo telling him of all the events that transpired under Mordack, and meeting the Royal Family of Daventry, and Jollo filled her in on all the details of what transpired while she was gone (though probably could have taken less than a week, and likely no more than a month). Shadrack's letter is dated about 1 month before "present time". After which she was forced into 'seclusion'. Alexander traveled for three months before "present time". So technically, all the periods put together is between 6-7 months give or take.
Those references start to get a bit recent for a medieval timeframe, at least for me. But that's personal opinion, I guess. And yeah, I know there's always been some fudging on that, particularly with Shakespeare, but still.
Come to think of it actually, there are several more real life "recent" references in KQ4. Several of the tomb stones have the names of real people on them. One mentions the boxer Simon Byrne mentioned on one for example was a real boxer, who killed a man in the 1800's. Another tombstone mentions seidlitz powder which was a brand/product name from the 1800's, the city of Chelt'nam and its waters, a place in England known for a mineral spa. Count Dracula in KQ2 (most certainly inspired by his Victorian era counterpart from Bram Stoker's Dracula, set in 1897, even down to the Bela Lugosi clothing)?
The stuff in the fisherman's shack in KQ4, is pretty much late 1800's to early 20th century technology, the stove, the cans of food, the coffee pot. The inside of the gingerbread house in KQ1AGI, is also late 1800's, early twentieth century, as is the inside of the Woodman's cottage. The water pump outside of the woodman's cottage in both versions is late 1800's, similarly there is another one of those water pumps in KQ8 in the Swamp. The inside of Grandma's house is also c. late 1800's in furnishings. Almost everything in the haunted house in KQ4 is also late 1800's in furnishings. Actually even the log cabin style of the woodcutter's house (although appearing less modern) in the remake is late 1700's-early 1800's in style if I remember my architectural history correctly .
As an archaeologist/historian maybe i'm trained to spot these differences more, but there are many things that are not "medieval timeframe" in origin. But as someone with some understanding of this stuff, they certainly glaringly 'modern' to my perception (perhaps more so than even the occasional historical/fictional name drops). Since these are kinds of details that give a form of relative dating when trying to figure out the age of a site in real life.
Seriously though, you must really dislike the real-world references in fan-game KQ2+ (Elvis presley, Teddy Ruxpin, George Lucas, etc). That game is spilling out at the seams with real world references.

:p.
Touche. I'd forgotten that Valanice mentioned Rosella was nearly 20 in the intro. So yeah, that does compress the potential time between 4 and 5. Unless 7 is actually a prequel, since I'm not sure there are any references to 5 or 6. It could have happened before, or even simultaneously with 6. Of course that's going by the strictest letter of what is mentioned and not mentioned as the spirit of the game is pretty clearly a direct sequel to 6. And you really do have to take the spirit of the games into consideration too, otherwise the letter of what happens becomes meaningless.
Well I think this clearly causes some issues, I think the whole point of why Valanice is wondering when Rosella is going to get married, is because her brother was already married. At the end of KQ7, Rosella decides she will stay in Etheria for a time (who knows how long).
If you place KQ7 before KQ5, it kinda destroys the build up to KQ5-6, which doesn't show Rosella seemingly dating anyone (Rosella obviously loses track of Edgar at the end of KQ4 as she is in a hurry), and Alexander appears to be the first to do so.
We also know obviously that the developers had intended the series to be chronological in nature (obviously if a fan chooses to ignore intent, it is clearly unofficial, and that doesn't make their reinterpration "correct", the developers intended way is the "correct" way). Intent is very important. Based on on the family's comments at the end of KQ6, it seems they were too worried about their son's safety over the three months he was missing, that Valanice probably wouldn't have been thinking of trying to hand off her daughter that soon. Though I think KQ7 does occur shortly after KQ6, so that they would occur "about the same time". Still the time between KQ3 and KQ7 is limited.
And Graham has gone gray.
BTW, you mentioned the 'gray hair' in KQ5, Actually go back and play KQ4, Graham's hair is an even lighter shade of gray in that game, than in KQ5. But obviously, KQ5 isn't set before KQ4,

. It's also pretty darn gray in KQ3 as well.
See Here. (KQ4)
and here (KQ3)
Though with IA's KQ3's rewritten plot (with the extra months/year between KQ3-4), you could probably set KQ5 between 3 and 4 chronologically (during that "get to know each other period), and still get a timeline that would "work" within its retelling. KQ5 is pretty stand alone, and doesn't make any direct references to KQ4 (if you ignore the obvious backstory elements given in the manual, and in the game's menus). So ya a fan could probably pen a story with a convoluted order of 3, 5, 4, 6, 7 or 3, 5, 4, 7, 6 and still make a narrative work (but either way its very convoluted)

.