Well you asked how many of the weapons came from MOE? Most of them LOL. Obviously the Magic Shield, Slingshot, Dagger, Axe, Shining Sword, and a few other items are in other games in the series.
Well actually the victorian era furnishings, and William Shakespeare book makes sense if you consider the fact it was lived in by people of Innsmouth, Massachusetts, as early as the 1500's or so (going by dates on the gravestones) and they lived in it until around the late 1800s or early 20th century, before it was abandoned except by ghosts.
One of the earliest gravestones mentions a date of 1546. It is for a small boy named Willy, and he is infact one of the ghosts that haunts the mansion.
Granted that date makes an issue with Lovecraft's history for insmouth in that he had the founding of the town in 1643, so the house or at least the graveyard predates founding of the town.
Granted its possible that the mansion was built before the founding of the town proper, or that the ghost is haunting the mansion but not necessarily ever lived there.
Its also anachronistic in that the date predates all the major settlements by colonists in new world by at least 100 years. However its possible that the mansion had been built in england before being moved to the Colonies at a later time. Which might make sense since several people in the graveyard had died in England.
A noteable unnamed individual buried in the graveyard includes a man who died from a fight with the famous irish boxer, Simon Byrne (historically the gravestone matches the one for
Alexander McKay). According to the companion the gravestone was written for a man in his strength.
In another grave was a woman and her daughters that died after drinking Chelt'nam waters(Chelt'nam, or Cheltenham is a location in england), she believed if they had only used Epsom salts they woudn't have died (Epsom salt derives its name from a town in England of the same name, where the salts were originally prepared).
Yet another person died by bursting from use of Seidlitz powder(Seidlitz is a village in Bohemia, and the powder is named after the natural water found near there).
Now in King's Quest companion, we learn oddly enough that Mr. Kurtz from the story Heart of Darkness who died in Africa, was ultimately buried in one of the Ooga Booga cementaries. His famous line, "The Horror The Horror" engraved on his epitaph.
Remember its essentially our world, just our world where magic has existed. Like our world it shares all of our same planets as well. Everything that has happened in our history is part of their history as well(don't ask me to explain how that works out exactly, but historical events and inviduals get mentioned throughout the series).
People from ancient Egypt built a crypt in Tamir, in order to hide Pandora's Box, and bury a mummy. The Innsmouth citizens found the key and hid it in the mansion.
Manannan has a copy of Philosophies of Socrates, and Ancient Arabic Mythology in his library

.
Cleopatra's soul was taken before Samhain.
Roberta once said in a chat I was part of that her games happen in our world but at some distant period in the past...
Granted it appears she never really thought about how they fit historically, so anachronisms exist, and there are obviously geography issues. But eh we are supposed to have a certain suspension of disbelief.
I also don't know if she intended for Daventry and Kolyma of the games to be the locations of the same name that exist in our world or not, but who knows.
If she intended Kolyma to be the same place, then it would have to be in a time when that part of the eurasian continent was located in a tropical latitude before it drifted to the current location where we see it today. On the other hand the concept that world or weather can be altered or controlled through magical means also exists, which could also explain Kolyma's "tropical setting".
On the other hand KQ3 kind of implies that Daventry is in the Sierra Mountain range, somewhere near Yosemite valley, as Half-Dome can be seen, and Alexander comments on seeing it(granted he also admits its probably because he's in a "sierra game"). Breaking the 4th wall and all that...
As a side note its not uncommon for authors to take the idea that magic exists in our world, just most people have forgotten how to use it, or because it can't be tested people deny its existence.
H.P. Lovecraft wrote his books as if they were set during early 20th century in our world. In his stories magic exists in our world as well, but usually magic of the darker sort derived from dark beings from the cosmos, called the Old Gods.
Of course Harry Potter also has the idea that magic exists in contempory times, but people who deny the existence of Magic, or cannot use magic are muggles. Magical beings and most magic users have moved to remote places of the Earth to practice their crafts.
I don't know if you have played Gabriel Knight series, but it also is a story where magic and the darker creatures of myth exists on earth.
The Princess Bride was supposed to have taken place on our world, in some little known kingdom of Florin. However refrences to real world locations are made in the story, such as Sicily, England, and America.
Its literally quite common to have magic, and monsters take place on earth in stories. As well as claiming the existence of "little known nations" or towns. The authors just expect the audience to have a willing suspension of disbelief.
Even in political thriller shows and movies its often common for some ficitonal, so called, "little known third world south-american,
european, or african nation" to be mentioned as the plot of the story. As well as in other stories.
Princess Diaries has "Genovia" for example.
Here is a shot of the outside of the Old Gnome's Shack in Daventry(sorry there are no inside shots in the game);

Here are some outsides of locations in Serenia, I'll add inside shots later(when possible);
Swarthy Hog/North Inn;


Baker Bros. Bake House;


Serenia Towne;


Crispin's House;

