[SOLVED]
The above method didn't work in my case. Still stuck at 17.7 Mb.
I was going to give up then this crazy thing came to my mind.
1) I booted from a El Captain working OS and connected the Sierra beta HD as an external drive.
2) I launched the sierra beta 2 upgrade files from El Captain TARGETING the SIERRA BETA DISK [CAREFUL HERE] one by one.
Installation proceeded smooth. and reached the end.
3) shut down the system, connect SIERRA BETA 2 freshly upgraded HD and had a welcome screen, fiiled it with my information and BAM!
no more upgrade request, system is up to date.