If Schala is as strong as Zapar is implying, Schala should be able to defeat Lavos on her own. She can't. End.
Schala said IN THE GAME that she would use the LAST of her pendant's powers to teleport the party to safety.
If, as is being suggested, the pendant enhanced her powers, then once it was drained, she wouldn't have had nearly the energy to last long against Lavos. Furthermore, it is equally likely that schala actually died or came very close to it when the Ocean Palace collapsed on her. AND inside the DBT, it is possible that magic as it was known in the CT world cannot exist, due to the non-existence of Lavos in its original form.
Sealing a time portal shouldn't really be that difficult. Its more like a forcefield, I think. If the portal is a gateway, then it is possible that a barrier on one end would appear on the other. I think it is the same kind of barrier technology used on Melchior and, perhaps, on the party and the mammon machine.
We do know that over time the pendant can lose its power. This is proven by the fact you have to recharge it at the mammon machine.
It is also said that when Schala communes with the mammon machine, her pendant starts to glow. It is conceivable that using magic and enhancing it with the pendant, as opposed to simply opening seals, drains it much more quickly. Needless to say, activating the mammon machine would have required incredible energy.
Even IF Schala does have near-god like powers when her pendant is fully charged, she is still human, and subject to exhaustion and injury.
To draw any conclusions about her simply because she was unable to stop from becoming part of the Time Devourer is ludicrous.