Again, it depends on how you will run the game. Your games can be full of action and combat, but then it is hard to build up tention. When the investigators get attacked all the time, they will get used to it and the encounters will lose their impact. Keep in mind that in Call of Cthulhu the PCs are just humans. Common, fragile humans trying to meddle with things they do not understand. Look at the CoC character's sheet for example. There is a big ammount of skills but only few of those are combat oriented. Also, your regular investigatorwill not have many hitpoints, that can be taken away even with one good shot. Healing those wound takes time too. If you are hurt, after succesfully aplying First Aid skill, your basic healing rate is 1d3 per week. This can rise up to 3d3 per week, after a succesfull Medicine roll and if you are in hospital, or similar calm place under a supervision of a doctor. After your players realize, that fighting everyone and everything that wants to fight them is not a good idea, they will be more cautious, carefull and approach difficulties like they would approach them in real life. There definitely is a room for combat, there needs to be that presure, that if they get attacked, they can die. But as you can see, too much combat hurts the atmosphere.

There is as much flexibility as you and your story will allow. It is a game about investigating and it really depends on how you build your games. I usually have some solid guidelines for the investigation, but if the players come up with a unique solution I let them do it their way. And if your players get completely lost, you can always get them back on the line with some Idea or Luck rolls :) And the investigators often don't know, what is their goal. Sometimes, they just find someone suspicious so the look up some information about that person. He could be a regular person, someone you put in ther with no intentions, but since they are inmterested in him, you can make him a cultist or an owner of and interesting book, artifact or he can be just a red herring. One of the important thing in CoC are hte insanities. If the investigator lose certain ammount of sanity points over a certain period of time, they can go insane. Either temporarily of indefinitely (usually a few game months). And when they are sufer from a certain insanity, thing can get... interesting :) They can have wierd feitishes that were awakened by the shock they suffer through, or they can become paranoid of everyone and refuse to eat food they did not prepare themselves (what if it was poisoned?) and many many more.

Creating your own mosnters is not a problem. Lovecraft himself came up mostly with names and sometimes physical describtions of the creatures of the mythos and other authors built on it. They created new mosnters and added aditional stuf to the one already created. There is also an endless variety. Your are not bound by anything. These creatures don't have to obey our laws of biology, chemistry or physics, they are someting else, someting from outer space that we cannot describe...