at least one person would see a giant tick pop out of the ground and live to tell about it or at least feel the earthquake he would cause by doing so
That is, why did he just go back to sleep? He didn't after Queen Zeal awakened him in 12,000 B.C. So why is it that no one in Guardia seems to have even heard of Lavos?
I dont think the Lavos Summoning ritual was meant to 'draw' Lavos out of the ground, considering Magus was playing with Time/Space during the ritual (I say that because there would be no reason for the gate explosion if he wasnt). Instead, It would very likely be to gain direct access to the core or the shell without having Lavos notice his attempt. It would be a lot easier to send someone to Lavos instead of moving the 'fat giant tick' back onto the surface. In fact, I dont think Lavos even knew about the attempt in the new timeline.
The only other true summoning ritual I noticed in the game would be Dalton and the Golems. Looking at that, Summonings would be gateways or portals that would allow creatures to step through.
If this were true though, why would Magus need the gate that was created after the battle with Chrono to return to 12,000BC
Whatever this ritual was, Magus was spending a lot of time preparing it; and it had a high chance of failure it seems if even Magus could not keep it under control. If this was indeed his first attempt at opening a gateway and 'summoning', perhaps he was either too weak to try again, or perhaps he did not have the time to try again. Remember, he probably could guess that if Frog was alive, the group would probably still go after him and finish him off.
The question this would raise is how Magus knew about the portal appearing beneath the Tyranno Lair. Now, thinking that Magus had enough time in Zeal to enact some major changes as the Prophet, Here are two theories.
1.) Magus made the portal beneath the city and escaped before Lavos fell, knowing where that exact place would lead him when he made the portal.
2.) Magus studied up on his history OR Janus knew about the 12,000 BC portal. Either way, Magus could try and find the portal that leads back to the 12,000 BC portal.