Electronic – Which pins should I use to connect a watch crystal for powering a low frequency crystal oscillator in ATmega328p

You don't say what the device this is going into. However, if there will be a micrcocontroller present then you likely don't need a separate crystal driver at all. Many microcontrollers (and just about all Microchip PICs) come with low power watch crystal oscillators. You add the crystal and two caps, and the micro does the rest.

Yes, driving the crystal with a sine wave is the best for the crystal.

The datasheet confused me a bit because section 8.4 makes it seem that I place the oscillator between XTAL1 and XTAL2, while 8.8 suggests that I connect it to XTAL2 with XTAL1 not connected`

The microcontroller has several clocking source options (internal or external).
One of them is by connecting an external crystal to XTAL1 - XTAL2 (section 8.2)

Another clock source can be using an external oscillator (this means an external device that generates the clock pulses) which can be used like

You can use one or the other, not both of them.

Regarding your question about the UART you should check section 19.11 which has several tables showing the error percentage for a given baud rated and crystal frequency used.

For your specific case with a crystal of 3.6864 MHz the table that shows the possible baud rates and the error percentage is

and it's perfectly usable with 0 % error in all the common boud rates.