So first let's consider the set of sources you linked. They are mostly historical chronicles from the early 12th century, reporting on a speech/sermon by Pope Urban II to the Council of Clermont in 1095. Essentially, we are reading after-the-fact accounts filtered through two layers of propaganda or, really, marketing. Urban wanted to convince those present at the Clermont of the need for crusade. The chroniclers wanted to portray the endeavour in a positive light.

In other words, these are not strategic plans or administrative documents. This is JFK saying, "We choose to go to the moon," not NASA arranging contracts with NAA and Grumman.

So what can we know about the prehistory of the crusader states?

Contemporary discussions about the motivations for the crusades emphasize the need to "liberate" the Holy Land. The terms libertas and liberatio used in the sources aren't just philosophical or ideological like we think of them today. In the 1090s--right in the mix of the struggle over church vs secular authority--libertas and liberatio had very concrete legal, political, economic meanings.

One purpose of the crusades was to liberate the Near East, specifically Jerusalem, from non-Christian rule. That would seem to imply the need to set up something in its place, right?

Certainly the people who heard the call to crusade thought so. Guibert of Nogent (one of the chroniclers who gives us a version of Urban's speech) wrote that some peasants and even lords were so zealous about the crusade that they sold off their possessions and land for far less than they were worth, anything to get a good sale, because they were going to the Holy Land and not coming back. Peasants packed up their entire households, including their little children, onto ox-drawn carts. That suggests that not only did they know they weren't coming back (always a fair assumption with medieval travel), they thought there was a chance of setting up a new life.

As to the Seljuq Empire, it was actually already dealing with all sorts of internal and external threats before the arrival of the First Crusade, really starting with the deal of the sultan in 1092. Provinces were at war with each other, territory was being leeched away in both the east and the west. (It's a pattern: Muslims chroniclers of the First Crusade attribute the crusaders' victory to the constant warfare among the Near Eastern city-states, which prevented the raising of a unified army that could easily take the Franks, who were terrible fighters with BARBARIC battlefield medicine, but shucks did they ever have nice weapons.)

In fact, the Byzantine Empire turned to Urban for aid because of a victory against the fracturing Seljuqs: it hadn't been enough. With the aid of the crusaders, the Byzantines pushed the Turks off some strategic islands and recaptured a good chunk of western Anatolia (modern-day Turkey), which had some important cities but also was a vital source for agriculture. They didn't get back quite all of their territory, but they did get some of it. Of course the Latins insisted on setting up their "Crusader states" in what had once been the Roman Empire. Relations were not always the warmest.