But a seal of the combined UESPA and Starfleet is shown in (ENT: "Demons") at Starfleet Command, which shows they were combined before the creation of Federation and not much later!
Actually, such a seal would usually indicate that one of the organizations is subordinate to the other. In this case, I'd rather think UESPA is subordinate to Starfleet Command than vice versa, as a "space probe" agency seems to perform a narrow slice of the work of Starfleet.
It might be that exploration is a relatively novel thing for Earth space services, since you can't really explore much before you get high-warp vessels. That is, you could if the space immediately around your planet of origin were unexplored, but in Trek this isn't the case as Vulcans are there to tell Earthlings that it has all been surveyed already.
United Earth might have originally had a large mostly military organization with assorted commands like Military Assault Command, Military Logistics Command and Starfleet Command. When space exploration became technologically easier and gained in importance, an agency or several attached themselves to Starfleet to pursue their goals with the help of Starfleet hardware. With time, the balance of power would start to shift, with Military Assault becoming insignificant and Starfleet Exploration becoming crucial for Earth's survival - and a mariginal organization like UESPA would become a very important one (much like the once mariginal Marine Corps has become vital for the United States through assorted foreign policy changes).
How serious can we take your canon when there are stories about Braga having a hissy fit over the TMP DE
It deeply saddens me to see that people devoted to following a literally fantastic show about an improbable future far away from home would choose to view that show through its "Earthly" aspects.
Many of the people involved in making Star Trek are assholes by definition - that's Hollywood for ya. Certainly many of the people involved in making TOS were. Why should we care? I've never met any of these personalities, thankfully. Instead, I've seen what they have produced. Much of it is satisfactory. Much of it relies heavily on the support of the larger framework of a well-built fictional universe, and wouldn't stand alone without that crutch. But I have nothing against those who need and use crutches, not when they can dance beautiful ballet with such an aid.
Whether a particular spinoff was produced by a philandering egomaniac, a bean-counting herbert, or a hopelessly nerdy team of fans is neither here nor there. The end result is rarely dictated by the nature of the creators, and instead reflects the times and the demand of the market. We are part of that market, part of that demand. We aren't any part of what "TPTB" choose to do on their working hours, let alone on their free time, and it really amazes me that any of us should care. Except for the devoted groupies, of course - a valid subgroup of followers to any movement, but probably not the one for which this forum chiefly caters.
Timo Saloniemi