Code P111400, O2 sensor issue?

There are no pulling issues, the car goes as normal in steep hills. The only feel I get that the acceleration has reduced is the lack of turbo pull at sometimes. It has reduced a bit.

Car really pulls greatly though, but as a daily driver I can identify the difference. The revving sound has slightly changed.

Yep, that code will cause the issues you have described. I would replace the sensor and clear the codes which will help the gas mileage come back around. This guide can help:

https://www.2carpros.com/articles/how-to-replace-an-oxygen-sensor

Bank 1 sensor 2 is the sensor closest to the engine. Please go over this guide and get back to us.

The mechanic said the O2 sensor was not available in stock, and I can drive without issues. But I don't like driving with an engine warning light and don't get the actual power and pulling of my car.

Can I still drive the car until the stock arrives? How much kms can I run in the meantime? Does it cause any other issues in the short term?

Also, I want to add one thing, from yesterday I felt my steering had become tighter than usual especially when running. It's less tight when the car is not running, but overall car steering had become tight. I've noticed this when I started car in the noon time after being parked directly under sunlight for the whole day and started the car when it was too hot.

It's occurred due to the O2 sensor issue & engine warning light? That have any relation. Or any other reasons related to the hot weather?

I don't think the oxygen sensor and the steering issue will be related. I would not drive the car at high speeds until you get the oxygen sensor replaced. The PCM will substitute a default value for the oxygen sensor, but it is not optimal.