TVS diode array for 3.3V I2C lines
1) The max standoff voltage is a guarantee of no conduction below that threshold. So there will be no clamping below 3.3V. Above that and the diodes will begin to clamp. Do not think of the TVS Diode as a switch that is on or off, it is a semi-conductor so it can partially conduct.

2) Max clamping voltage is the voltage at which the TVS Diode can conduct the most current. On the datasheet, clamping voltages are given for 50A and 100A and the graph, Figure 4 shows this over its working range. This is shows for an 8/20us pulse which is very fast, typically ESD events from human contact are slower (see this RS article on discharge rates. They spec to a 153ms event.). What this means is that the rate of change of current (di/dt) is lower, so the clamping voltage is lower.

3) I2C is a duplex communication protocol. Duplex means that it has two data lines, but they only operate in one direction each.

The question about unidirectional and bidirectional can lead to a confusing answer, as admitted in this ON Semi paper. Best to deal with single diodes as a beginner.