Can I measure the battery voltage in the DS12C887 to check the SoC?

Unfortunately there are no external connections to the internal battery of the DS12C887+ so it is not possible to measure the battery's State of Charge directly.

Can the internal battery of DS12C887 be charged?

The internal battery of DS12C887 is a primary Li battery and cannot be charged.

Is the life of the DS12C887 internal battery affected by the temperature of the external environment? Is the battery life shorter the higher the temperature of the environment?

Yes. Since the self-discharge of the battery is more intense at higher temperatures, battery life is shortened under such conditions.

Is the data retention period of the DS12C887 the same as the lifetime of the internal battery? Is the actual data retention period related to whether the DS12C887 is turned on?

The data retention period refers to the retention period when the device is powered by the internal battery. If an external power supply is connected, then the device is powered by the external power supply rather than the internal battery and battery capacity is conserved. In any case, the longest retention time is determined by the battery life.