So even i am in process to understand visa procedure and i found this to be guidelines -
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And this clearly states you have to book an appointment to give your biometric and photo taken which will be used to issue a visa. So unfortunately it looks like your mom has to visit the center in person, nahi to you get 15 days VoA which i don't think serves your purpose.
VFS is pretty much the sole agency appointed by Thailand for Visa purpose.