A flashing white light could be a number of things, such has can't connect to your Router / internet (even if the NOW App works on your phone) or it's a HDMI issue (but you mentioned you have tried the Stick on another television so i doubt that the problem).
Or the Stick is attempting a software update and is having difficulties.
Did you get anywhere when using your smartphone has a wifi hotspot (assuming you have mobile internet data on your smartphone) with the Stick to temporary bypass your Router to take this out of the equation.
No idea, i don't own an iPhone (maybe Google it ?).
if you want to watch NOW on your TV screen i believe you can use the iPhone cast button on the NOW App on the iPhone (only thing i am not sure if it's still broken or fixed now reading this link below).
If you can't get the wifi hotspot to work, then you could try a different wifi channel on your Router or if your Router is dual band, try both 2.4 & 5ghz.
Also to see if there is some conflict between the Router and Stick, maybe try a full factory reset on your Router.
Apart from that perhaps test the Stick on a family members or friends internet.
To activate the iPhone’s mobile hotspot do the following (you may already have done this, but just in case):
1. In settings/mobile data ensure “Personal Hotspot” is turned on.
2. In settings/personal hotspot, turn on “Allow Others to Join”
3. Make a note of the wi-fi password
4. Using your Now stick or other device, look for your iPhone name as a wi-fi network and use the password from step 3.
Hope this helps.
If you are powering the Stick from a USB port on the TV, try powering it from its mains charger instead, and see if that helps.