Probably not stripper clips. That would have resulted in fingerprints on at least one cartridge in each clip. More likely the magazines were loaded with cartridges that had been wiped clean, or handled only with gloves when taken out of the box.
Use of a Mini-30, if proven, might be a ruse to mislead people. The obvious first thought when 7.62x39 cases are found is to think AK-47 or SKS, suggesting Muslim or drug cartel operatives.
I’m pretty sure a proper forensics team with a pile of spent brass/steel could determine whether an AK, Mini 30, or SKS was used based on extractor marks and such.