I think there are a few things Trinh can do in her situation. The first thing I can suggest is that she make a forum on Blackboard where questions can be asked publicly, such as seen in our course. This way, if other students have the same question, they can refer to the relevant forum discussion. This will decrease some of the duplicated emails. Also I think she should be clear how she wants students to communicate with her so she does not have to check in so many places. She could encourage her students to use the forum unless its something they don’t want publicly posted, in which case she should give them ONE personal way of contacting her, such as her university email.
My second suggestion would be to use a live chat at the end of each guest lecture live-stream. I have participated in some live stream lectures with chat capability, which I found useful to clarify material or ask a question. This way the guest can answer some questions, taking some of the workload off of Trinh.
My third suggestion is to hold office hours where students ask questions via chat. Kind of like our Collaborate session over the weekend. She could also do it by video or audio but that might get a little crowded. She can then post the chat content so others can also look at it for reference. If students know there are office hours and it is convenient for them to ask questions during this time, it might decrease the number of emails she will receive.