Thank you for your question.
Regardless of the debate going on in the country as to the legalization of marijuana, at this time it is still illegal, including in South Carolina. You are correct that a simple possession is punishable by a maximum of up to 30 days in jail (not likely) or a fine up to $200. For a first offense, these are most commonly taken care of with a conditional discharge, as you alluded to.
With the conditional discharge, a defendant will be required to plead guilty. That guilty plea will not be entered into the record unless the defendant fails to fulfill the requirements of the probation. Once a defendant completes the terms of the probation (conditional discharge) they will be discharged and the charges dismissed. This will allow your daughter to later expunge these charges from their record. The court normally also orders that a person complete about 30 hours of community service as part of the sentence
, though since conditional discharge is technically probation, they are also under whatever other conditions the court may impose (e.g., subject to random drug testing, or they can be ordered to enter into outpatient treatment, for example).
With respect to a warrant, it's not necessary in this case because your daughter granted them permission to enter the room. Even if she didn't though they likely would have had probable cause
to do so given the smell of marijuana coming from the room. A person is not required to be read their Miranda rights unless they are taken into custody and the police intend to question them. If they are merely arrested and never questioned, Miranda is not required (though television and movies would have most people believe otherwise).
Generally, speaking it doesn't hurt to speak to a criminal
defense lawyer prior to her court date just to review the matter. Many lawyers offer free or low cost consultations. There can be situations that arise where officers did not act properly and then there may be grounds for discharge, or situations where due to extenuating circumstances, a court may not offer a conditional discharge.
If however, after speaking to a lawyer your daughter feels like "they have her" so to speak, she could simply plead guilty herself and a judge would typically grant the conditional discharge at that time.