It's always advisable that all 4 tyres be of the same pattern and size.
Regarding the numbers, there is always a logic behind designing tyres with a particular tyre width, aspect ratio and diameter. I am not that technical here to pinpoint why exactly 70 and not 80 for 185 section tyres.
Oh I see, So like I said above don't mix different size tyres. Get the same size as others and also same brand/model if possible.
As you already know, the first block of three digits is the width of the tyre measured in mm and the second block of two digits (i.e 80, 70) is the height of the tyre sidewall measured as a percentage (Aaspect ratio) of the width. In your case: 80% of 165 is 132mm and upsize: 70% of 185 is 129.5.
The sidewall height of the upsized tyre should be less then +/- 3% to OEM tyre size, otherwise the tyre will touch the wheel arch/well.
Now you can buy the OEM size and use it till other tyres wear out, then upsize all the 4 tyres and use one of old tyre which is in good condition as spare. Using different size tyre for limited distance is fine but it shouldn't be used as regular.
In 185/70 r14 size, there are lot of different tyre brands and models to choose. Continental, Bridgestone, Michelin, Yokohama & etc.
You drive on bad roads? so what? Where are good roads in India?
As a generic statement it holds good, but why people suggest such a weird statement in specific to few cars is because the rim width is different for Steel & Alloy wheels (of same vehicle, different variants) Alloys comes with higher variants (generally) and they have larger width compared to the steel counterparts.