Please find our comments below for your questions.
1) Your understanding is correct for the t6 parameter.
2) We have tested the part for a max of 30 pF load capacitance under 100 MHz and a max of 15 pF for over 100 MHz. We cannot guarantee any datasheet listed performance if the device is operated outside of the operating conditions. Since we have not tested the part we cannot say what will happen or how the device may respond, maybe the linear relationship will be maintained with some affect on the performance or as you say the PLL may lose lock but we cannot say anything with certainty.
Thank you for your answer.
My question has been cleared, so this case is closed.