Why did my idli looked cooked from outside but was still mushy inside?
If you are in a "perfect temperate climate" then the batter should not "go rotten" as the method is from southern India, which is very hot. It does not rot there, so it will certainly not in a cooler climate. You may not, however, have the same rate of success for the same reason. 

You imply you are an experienced idli and dosa maker, so you will have to experiment with fermentation and cooking times. If they did not rise, it would suggest you are not fermenting long enough. And denser batter will take longer to cook. You already have a genuine desi (Indian) recipe and methodology, you just need to tweak it to your local conditions.