I think there's some other bits and pieces of fluff spread out over the various books stating the slann started with armies of conjured up seraphon, but eventually made some of those build new spawning pools whenever they had need for more permanent forces or when they wanted to be able to fortify a certain place without having to constantly stick around themselfs. So both methods can coexist. Plus, making all seraphon genuine biological beings again would screw with the summoning mechanic (the hell are they coming from then…). So I'd assume both methods get to stay.
That wouldn't be difficult. If there's need, Slanns are still capable to conjure seraphon with their will. Starmemories fighting alongside real lizards
Well yeah, but at that point there's no point at claiming the armies are mostly normal biological creatures. I'd say the current mix is probably best where the memories form the bulk of their forces, and they occasionall put down some spawning pools here and there for more permanent forces.
...after all, when we play those shadowstrike lists at 1000 pts with no Slann, there must be some real lizardmen
There's some bits of fluff stating the summonend memories don't necesarly decay all that quickly. Sending out an army for a single battle, or even a short campaign should be possible. But yeah, at a certain point it should no longer be worthwhile to conjure it up with magic and just be easier to plop down a new spawning pool if all you want is some scouts to run around a certain area and clear it of nasties for the next couple centuries while you go off doing something more important.
That's from a White Dwarf magazine. It is in line with an official Facebook post that also explains how Seraphon can be beings of Star Magic but also can be regular beings that are merely teleported in for battle.