All of these points you raise are good, and for the most part, that’s why people are scared witless of greater demons, archdemons and princes.
They can do all sorts of nasty things. Such as dominate the wizard and force him to become a warlock slave. They also have access to incubuses.
The type I through III’s and the lesser IV’s can’t really do this. They don’t have the power, or the resources (e.g. hire an incubus)
The animages that summon things are called Summoners. Summoners and some types of druids/shaman work in a similar manner to what you describe; very much like Tom does. The way Tom listens for Gastrope’ or others to ‘summon him’ call him by name and then he opens a link through a nearby fire is pretty much the exact reverse of what a Summoner would do.
A summoner controlling a demon would have a binding on that demon, that would provide him the link to the demon, so the demon could hear him (much like Tom has) and he could use that binding (a very high level link) to compel the demon, but he, the Summoner would have to open the gateway for the demon (assuming it’s a lesser demon)
The real problem Summoners have is that it’s very freaking dangerous, you don’t have all the protections or armor that a wizard has. The wizard will build up all these spells, ritual etc to protect himself from the demon.
Summoner’s don’t really have this, it is shear will power. So Summoners that summon demons typically stick to Type I, II. Something they have the power to defeat in combat. In particular something called a Will War or various things (wizards have Wizard War) which are basically sheer tests of magical strength/magical wrestling if you will.
That being said, summoners can and do use Rune Magic, if they know it. Rune Magic predates modern wizardry and is probably the oldest from of formulated magic. You see Tom learning how to use it/read it/activate it in book II.
So serious Summoners would use rune magic to construct wards of protection, much like a wizard does with the pentagrams. A summoner can in fact make a pentagram inscribed with runes. If they are smart, they will.
Of course Summoners can summon about anything. They often use Summoning to summon animals, or spirits, or the dead. Remember there was some talk about this when Edwyrd was handwaving to Maelon about how to bring back Rupert.
Shaman’s do a lot of Summoning; although typically not of demons.
So back to your other point about making demons. Most humans would never even think of this because demons are elemental evil in their mind. They can distinguish humans from nearby worlds on the Astral Planes, but as the distance between planes increase the distortion grows.
Orcus was essentially creating demons out of his greatest Orc warriors upon their death. There were great ceremonies where the great warriors were linked to orcus and tattooed with various magical runes and then if and when they died, Orcus and his D’Orc Shamans would summon the warrior using the tattoo’s and links to capture the soul of the warrior and drag it into the Abyss and create a D’Orc body.