Can a Master issue a STOP condition regardless of whether or not a Slave has issued an acknowledgment indicating a completed transaction?

The STOP can be issued anywhere the Slave is not driving the SDA during SCL high. It may not be appropriate to do so in terms of completion of a message. But ACK and completed transaction do not belong together in I3C.

Can I3C Hot-Join Slave devices be used on a legacy I²C bus?

Only if they have a way to turn off the Hot-Join feature. Hot-Join is not compatible with I2C masters, so Hot-Join would have to be disabled for the Slave to be used on a legacy I2C bus. The disabling of the Hot-Join feature should be done via some feature that is not part of the I3C protocol (i.e., not via the DISEC command), since an I²C master does not support the I3C protocol.