When Eve and Izzy are to go back 4 hours in time to warn Gavin about the danger in the cave, others want to use the portal to take them back home while Eve and Izzy were gone, but James tell them that portal cannot change space/ time location while someone has gone through it, otherwise they will not be able to return, thus everybody decided to wait for Eve and Izzy to return. Well, one Eve and Izzy have indeed gone back in time and had successfully warned Gavin and then Gavin could make himself and all other safe from cave reptile, then the time line would have changed, and Eve and Izzy would then not require to go back in time to warn Gavin of anything, thus the duo would not have been required to come back through the portal. The entire time line of Gavin getting hurt by the cave reptile would have been erased, and Eve and Izzy would have come with others in the normal single flow of time line. Thus, James needn't have kept the portal open to wait for Eve and Izzy's return through it. He could have immediately switched its time/ space location to start sending others to home.
When Eve and Izzy come back in time four hours to warn Gavin, and they save Ty and Levi from James who anyway runs away taking the fuel rod, Eve and Izzy found that the portal is now much shorter, meaning it is closing. They still insist on informing Gavin. Well, they could have told Ty and Levi about the danger in the cave so that the duo could have gone to tell Gavin, and Eve and Izzy could have returned through the portal while it is still open. No need to do everything by oneself.
Much is made of Izzy and Eve having to get back through the portal. However, they could have just stayed and walked to the cave normally.
Ty and Levi decide to return to the cave through the portal along with Eve and Izzy. Gavin gets Eve's message, takes precautions, and is not get hurt, and Eve and Izzy don't require to go back through portal to warn Gavin, so they don't return through portal, thus Ty and Levi don't meet them Eve and Izzy while they were gone through portal thus don't get to reach the cave through portal. There seems anomaly, but as was described in other plot hole, when Gavin doesn't get hurt, the entire time line of Gavin getting hurt is erased, thus Ty and Levi reach the cave normally with others, without having walked through the portal.
Eve and Izzy go a few hours back in time via the portal in the cave. Both arrive again at the deposit box (this time together with Ty and Levi). The box holds the map of the cave where the portal is located. To open the box with the map of the cave they need a key. This key was left on the box the first time when they went to the cave. However Eve opens the box to write a warning message for Gavin on the map. But she couldn't have this key because at that time (some hours back in the past) it would still be in posession of Gavin. Therefore she couldn't write a warning message on the map to save Gavin. When they all 4 return via the portal to the cave they arrive some hours forward in time. There they found out that Gavin has received the warning message. Which could not be possible.