Don't fly charter airlines with one plane per route and that one plane is available only if all the other flights they use it for on that day are on time, is the best advice.
Otherwise they will quote their terms and conditions that say they have to get you there and back, not at your time preferences, but theirs.
You arrive the 8th and leave the 18th so that's what they will say.
People book this and similar airlines because they are sold as packages that must make all vendors mere pennies per customer. So when they want to optimize things, they will. Hence the POP stopover. There are not enough people in POP on the return to justify a separate aircraft, or they are not flying TO POP that day, hence doubling up on the way back. At least you are on first and the flight can't be very long.
I am sure there are scheduled airlines that fly this route that are more reliable but cost more.