You will have a hard time balancing the spice levels. You can't satisfy both Indian and non-Indian demographic or you will have to have a spicy, less spicy type of each item. As an Indian, I cant stand the westernized non spicy versions of the supposedly authentic Indian foods. So I wouldn't buy them nor eat in those type of Indian restaurants. Its the same case vice versa. Non Indians may not like the spicy versions…
In my experience non-westerners tend to underestimate the level of spiciness that westerners can handle (particularly westerners who are culinarily adventurous), and overestimate the average spiciness of their cuisine. If it's too spicy for me, it's too spicy for 99% of the population of any country, but it's damn near impossible to convince servers of that.
Personally, I find worst-case behavior rude, offensive and downright childish; and more annoyingly, it's generally acceptable behavior. It's sufficiently annoying to stop me from serving spicy foods by default, if its to a non-asian. And worst of all, they always think they can handle a decent level of spices, so theres no warning before they start huffing and dying at the table.
I imagine most restaurants aren't as annoyed by it as I am, but I'm pretty sure its primarily that worst-case behavior that stops them from serving properly spicy food to westeners. At least with an indian, if they can't handle their spices, they'll drink their water and shut the hell up.
No one enjoys seeing someone huffing and puffing at the dinner table