yes, i did have an exclusion order back when i was dating my now husband. my exclusion order ended way before i applied, though there is no rule against applying outland if an exclusion order is still valid. generally, i believe what happens is the visa office just waits until the exclusion order has ended before issuing the visa.
i believe your daughter probably just got one of those border agents that likes to scare people who don't know what they are doing. generally, the reasons someone is denied entry is because they can't prove their "ties" to their home country, which generally is ONLY proof related to housing and employment, or there is suspicion of working illegally. CBSA doesn't care about belongings, family, cars, or bank accounts, they want to know you have a place and/or a job to go back to.
it's far easier to cross when a pr applicaiton has been submitted. all the border agent will do is confirm it's in the system and let you in. if the application has NOT been submitted at the time of entry, then it's recommended to travel with copies of EVERYTHING to show intent on applying. they like to see you are following the rules, not making them up as you go. Also, again, proof of ties to the us is essential too, espcially if someone's been red flagged an will be pulled into secondary.
the border agent isn't going to know when your daughter leaves and enters canada again, unless he happens to be the same agent she gets on the next entry. if they really wanted to track her entry, they would have given her what's called a "visitor record" and require her to "check out" on her way out. if she didn't have luck there, then try another entry point. either a different airport or land border. you don't have to enter the same place every time. If that officer was really suspicious of your daughter, he would NOT have given her a stamp for 6 months. he would have shortened her stay, tracked her with a visitor's record or denied her entry. as long as she's prepared and does not talk as though she is moving to canada before approval, she will be fine.
the fact she has a passport stamp until april 30th, means they allowed her to enter for 6 months. this isn't a visa, it is merely a passport entry stamp that says she needs to leave BY that date. this doesn't mean she can't go back and forth between now and then. actually, from my experience, the fact that she's already been approved to enter until that date and has a future date stamped on her passport will only help her enter the next time, as long as it's before that date. Having a future stamped date in my passport was the ONLY time in the past 2 years i didn't have to go into secondary, they let me drive right in. in my opinion having that dated stamp for 6 months from now limits her chances of being denied at the border. once she's back, and she wants to stay long term, she can apply to extend her visit prior to that date stamped on her passport. Or she can continue to travel back and forth. When doing this it's important to keep the trips to 1-2 months at a time, with long term stays at home in between, and being prepared to prove ties to the us upon entry.