I have never checked on e-visas in details, but only Business visas are extendable for 6-12month. Agencies can also get you paperwork to cover the work permit issue.
yes, but people want to stay in Cambodia that they become fussy, no problem if you go OUT/IN, but if you go into Cambodia and possible extend visa in Phnom Penh than they might get a bit fussy, because any extension is a sticker for itself.
The Embassies can accept Non B applications, but paperwork will be send to Bangkok for approval, that takes around 3 weeks while the applicant has to wait in Laos. I think for Non O n ED the applicant has to return home country.
do not blame the agency. It is a business model and all people are free to choose. BTW we do not sale such packages, but certain group of people are happy to pay for such kind of visa.
in case denied entry it depends on various factors to which place they send you. You are put in the basement, passport with Immigration until they find a solution.