Electronic – Floating Differential measurement

There is virtual zero benefit in using differential inputs for your application. Assuming the sensor is close to the uC, you'll be able to get almost identical results from either single ended or differential, +/- a couple LSB.

The real benefit of differential signals is in long cable runs and noisy environments, for increased common mode rejection. They're also very useful in instrumentation, when dealing with very small signals. A common way to deal with sensors that vary their resistance in small amounts, such as strain gauges and pressure cells, is to use a Wheatstone Bridge.

^{simulate this circuit – Schematic created using CircuitLab}

This configuration by itself centers the measurement in the ADC span. \$V_+-V_-=V_{differential}\$ It's important to match the default resistance as close as possible. Any deviation from the default resistance will show up as offset in the measurement.

Again, this is done because:

1. the changes in resistance are very small
2. more often than not, the sensors are far away from the data acquisition and processing equipment

...as far as i have read about them (also their data sheets), if an ADC has both single and differential ended inputs, we can use it as a single ended input ADC OR we can use it as a differential ended input ADC. Which means we cannot use it as both single and differential input at the same time.

This is not always true. For example, I have used ADS1015 on a couple of projects recently. On this chip, whenever you switch the channel being read you also have the option to switch between single-ended and differential measurement. (This is not an endorsement of this chip for your project. Just an example of a chip that doesn't have the limitation you thought was universal)

Furthermore, even if you had a device that had to be configured as single-ended or differential for all channels at the same time, nothing prevents you from using ground as one of the inputs to a differential channel. So you could just configure it as differential and move on with your design. The only thing you'd lose is the opportunity to use the 4th input pin for some other purpose.

Another option, if you plan to use external signal conditioning, you can do differential to single-ended conversion in the signal conditioning circuit, and your ADC will never know the signals are anything but single-ended. This essentially makes the amplifiers shown in your diagrams to be external devices rather than internal to the ADC chip (and add some filtering in their feedback networks to reduce noise).