I am slightly confused on the use of the phrase "at least" when used in different contexts with regards to time...
My first example would be the following:
"I will be gone until at least 8:00", this sentence suggests to me that the person will be back at 8:00 at the earliest but it could also be any time past 8:00.
My second example would be:
"I will be home by at least 8:00", this sentence suggests to me that the person will be home anytime before 8:00 or 8:00 at the latest.
I have heard this phrase used both ways by people, are they both correct to use and if so have I interpreted them properly?
I would say #1 is commonly used to mean what you think it means.
#2 is unnatural and confusing. As you say, 'I'll be home by 8 at the latest' is clear and unequivocal.