Corrected entry: When they are running from the lava, and see the boat, it is already in the water. Wouldn't it have melted by now with all of the acid in the water? The fish should have dissolved too, surely?
Corrected entry: In the bar Paul calls Rachel "Linda", which is the actress' real name.
Correction: Paul says "Before we take off, I want to say thanks for the hospitality, Mayor Wando." But his pronunciation of Wando sounds like "Wanda". So he did not call her "Linda".
Corrected entry: When the pickup truck crosses the lava, the engine would have overheated and stranded them on the lava given the amount of heat going directly up into the engine compartment.
Correction: Actually, lava in movies is typically over-rated. Though the tires definitely would have been goo after one spin in that lava, the surface of the lava would be much cooler. You may actually be able to walk barefoot over it, it wouldn't be too much different from hot coals except for the much hotter liquid beneath. They spent 1 minute and 20 seconds on the lava surface, which would not have been enough to overheat the engine.
The tires being on fire would have increased the temperature under the hood even if the lava itself didn't being that the wheel wells would have also been capturing the heat within them.
Correction: Also, since the rear axle broke through the surface crust, the truck would become a rocket, due to the intense heat causing the gas fumes to ignite.
Corrected entry: Rachel calls a town meeting for 6 p.m. to discuss evacuation, about 8 hours after the danger became apparent. Wouldn't it have been more responsible to call the meeting earlier in the day, to keep the town on alert to get the hell out of there?
Correction: Paul has shown in many different instances that he believes the volcano wouldn't erupt and ended up being wrong about it. Added to that, he was also claiming his own, personal belief to Harry and that's basically why Harry couldn't do anything about it, even though he knew that Paul was wrong, Before that, it wasn't until Rachel and Harry discovered how weird the water was that they saw a real threat and then considered the evacuation of the town. It wasn't that they didn't care or were stupid, it was just how Paul felt and not seeing a whole lot of threats to the upcoming eruption. Also, the actual eruption came at a time they were not expecting, meaning that they knew/felt the volcano would erupt, but not exactly when.
But why only listen to what Paul had to say? Even if they were wrong and were looking at all the signs, wouldn't they at least be on the safe side and evacuate as soon as possible? It's like you said, Paul was wrong.
Corrected entry: When Grandma is being carried through the woods (after walking the boat through the acid lake), the far shot shows her to have shoes on. The close-up, however, shows that she hardly has legs, much less feet.
Correction: Look at the shot of her legs frame by frame - if you stop at one of the frames right before the camera cuts back to Rachel, you can clearly see that Ruth still has legs - they're just bloody and raw from the acid. The way the fabric of her pants is gnarled, when seen at full speed the camera moves and cuts away so fast that it does give the impression of missing limbs.
Corrected entry: It's amazing how the helicopter flew to the town in the ash without any problems, then suddenly gets its engine shredded from the same ash.
Correction: The danger for the helicopter is that ash could block the air filter, overheating the engine and wrecking it; moreover, the heli was quite overloaded and its engine had to work harder with a cooling system that didn't work well or did not work at all.
Correction: The boat is on the pier and Harry pushes it off into the water just before they get in.