Easy. Malaria was a farming country, farming requires physical labor, too much physical labor can lead to a curvature of the spine especialy in children, also if the curvature is ignored for too long it can worsen and eventually develop into a hunch. Seeing how the Igors all look quite young (most likely 20s to 30s) and the clouds had been around about the same time, it possible that because of the economic crisis that their curvatures went untreated and became hunchs. The bad news with this theory is that it's most likely that all of the Igor's families sold them to the various scientists for money and (possibly) allowed the hunches to develop.
Also in account for the blind orphans (a bit of a longshot), Malaria has had no sunlight for years, most likely that some folk developed sub-terrain blindness (seen in many creature who live in low light areas) and seeing how along with failed crops, not to kindly politicians and scientists tinkering with death machines every year - a lot of children can lose their eyes and their families simply for being in the wrong place at the wrong time.
Actually it is specifically for blind orphans, plus Rule of Funny.
Blind children require specialized care, so gathering the ones who lack parents from all over the country into a single facility, with an appropriately-trained staff, is probably more efficient than scattering them among however-many non-blind orphans' homes exist in Malaria.
Radiation, teratogenic chemicals, strange energies... mad science just keeps on giving and giving! And, of course, the Igors are more likely to have deformed children because they get higher rates of exposure.
There aren't necessarily lots of them - there are just lots of them in the movie. There's only 13. We see lots of sighted people in the crowds.
Precisely. If you're brilliant and morally or ethically dubious, you become a Mad Scientist. If you have a hunchback, you become an Igor. If you're normal, you're either a peasant or nobility.
Possibly some of the "orphans" actually have living parents, but were abandoned because they were blind. It's a culture where evil is glamorized, after all, and abandoning your kid because he or she has a disability is pretty evil.
