Do note that the jump drive is not the warp drive. I assume Amarr have had warp drive type faster-than-light or near-lightspeed drives before the first stargate to Hedion (i.e. before 21290).
As for systems around Pator, I understand that the canon is still that Minmatar had three systems, and were possibly preparing to do their own interstellar travel when Amarr appeared. I assume that this is the first interstellar nation found by the Amarr, and as such, they were unprepared to take on multiple planets at once, and proceeded to raid the main one. Slow Amarr advance may have allowed the Minmatar to finish their systems, possibly augmented with captured or scavenged Amarr technology.
Thus, the Minmatar have two avenues of expansion around the cluster: Primarily, being taken to other systems by the Amarr, and secondarily, flying there under their own power (or ships under new ownership), fighting a guerrilla war. Bigger warp-capable ships (such as cruisers) might be capable of interstellar travel without gates (not possible ingame to warp from system-to-system, of course).
On expansion of the empires: By some version of the fiction, the City of God was built into a central location in the Empire. Naturally, it might be that the Amarrians of that time considered all explored systems as their sovereign space (since there was no one else around to contest that sovereignty) and the actual colonized space was smaller. Aridia was explored (and the Ni-Kunni Reclaimed) around the same time as the Minmatar, so the Amarr seem to have been about at their current size (plus Minmatar regions) before CONCORD was founded. Caldari and Gallente had their blitz of colonizing systems during the war to keep the other empire from flanking the other.
Our surveys show that there are many planets capable of sustaining life in these systems, however the lack of a proper stargate network has drastically increased the travel time for our mission. I think it is worth it, I don't trust those Gallente further than I could throw one, and that isn't very far given their indulgences.
The initial speculation for a fall back position seems to be sustainable. However we have one potential problem, long range probes that we sent out have been reporting some strange readings. Our science teams think they are reading stargates and an adjoining region which is disturbing. I do not think it possible that the Gallente have yet moved that far and until we can confirm these readings and determine who or what these stargates belong to it would be ill advised to place ourselves in a position where we have our movement restricted on multiple fronts.