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Hi Z, I think recent updates to Hostapd and WPA_Supplicant have fixed the issues I was having with Stretch as I have it all working straight after the updates. I have one minor fix to make and will update the articles. Hopefully by the end of the week.
For your issue on Stretch it sounds like the Hotspot and Wifi are both active, i.e. Hostapd and Dnsmasq are conflicting with wpa_supplicant.
for this setup to work on stretch remove all the device setup from /etc/network/interfaces so it only shows the default 5 lines at the top.
then in /etc/dhcpcd.conf at the bottom enter interface wlan0

Wpa_supplicant is now run with the dhcpcd service but the entry in the interfaces file also runs it.

If that doesn't help then check back later this week and the full setup will be posted plus the extra tweaks to the scripts.