This question is in regards to the anime. I've watched all of the episodes that are available, so I do not mind spoilers.
Since humans who can turn to Titans at will (I think they're called shifters) can be cut out of the Titan body, can the humans inside other "normal" titans (like the ones who became titans via injection of spinal fluid) be also cut out and saved?
No, it is not possible. During Hange's experiments with Sonny and Bean [chapter number required], she mentioned slicing open the necks of titans in prior experiments. She was never able to find anything inside (in the anime an empty cavity was seen).
well in seaon 3 hanji's experiments when eren lost control and ate the tower, then when his butt was hanging out and had no muscle below the waist. When it was time to cut him out it was hard because he was partially absorbed into the titan(Mikasa had to cut him out after hanji wanted mbilt to sketch it) this is what happens to regular titans the humans in them get absorbed except the spinal cord.