A pressure of 600 kPa (87 psi) is relatively moderate, so the combustion properties are similar to those at atmospheric pressure where the autoignition temperature of hydrogen is 585°C. 

Vaporization of a trapped volume of LH2 will lead to significant increase in pressure due to the very large expansion ratio as the liquid converts to gas. Relief devices are required since the pressure increase is likely to be far in excess of the pressure rating of the system. When vaporized as part of a flowing process, pressure will not increase. As the LH2 is warmed, it undergoes a phase transformation from liquid to gas (progressively more bubbles until 100%) at a constant temperature and pressure as it boils. After it is 100% gas, it will continue to be warmed until it reaches the desired temperature, usually ambient. The vaporization occurs within a closed system and only hydrogen is present. No air enters the system and the volume of liquid removed from the LH2 tank is replaced with lower density gaseous hydrogen.
 

This is an impossible question to answer without greater understanding of the quantities of hydrogen involved, the types of vessels involved, and the atmospheric conditions. Several companies offer software to model such releases. It’s important to note that there is a high probability of ignition either during the vessel rupture or from nearby ignition sources.

The National Fire Protection Association (NFPA), the Compressed Gas Association (CGA), and the Society of Fire Protection Engineers (SFPE)   represent the U.S. fire protection and engineering community, and these organizations publish handbooks and standards/guidelines that describe the properties of hydrogen. There are many other organizations and documents that provide similar information. The Center for Hydrogen Safety within AIChE has training material for the properties and safe handling of hydrogen at the following link:

Fundamental Hydrogen Safety Credential 

It varies slightly due to different density of LH2 at different temperatures, but a gallon of LH2 at atmospheric pressure (0 psig) is ~113 SCF of H2. The expansion ratio is about 840:1. In metric units, a liter of LH2 at atmospheric pressure (0 MPa) would expand to about 840 liters of STP of gaseous pressure.

This is not an easy question since many factors influence how much hydrogen can be transferred from one vessel at a higher pressure to another one at a lower pressure and the rate at which it can be transferred. The pressure in the higher vessel will fall while that in the lower vessel will rise as gas is transferred, so the flow rate will typically slow down and eventually stop as the pressures equalize. Fueling protocols have important upper and lower bounds on the flow rate or pressure change rate that is allowed when filling a vehicle. The SAE J2601 family of standards detail those rates and ending pressures.  Most hydrogen dispensers use a cascade method of fueling (switching between multiple supply tanks) to maintain the desired flow rate and maximize the amount of hydrogen that can be supplied. As a general rule, a higher percentage of gas can be transferred from the supply tanks when filling lower pressure 350 bar tanks than higher pressure 700 bar tanks.  Similarly, as storage pressure Y psi is increased, a higher percentage of the gas will be usable for either fill pressure.

The density of hydrogen does not vary linearly with pressure since it’s not an ideal gas. For reference, the density of gas at 350 bar is 24 g/liter and at 700 bar is 40 g/liter. Thus, doubling the pressure only results in a 60% increase in storage volume. For this reason, 700 bar vehicle tanks are rarely filled from a single tank and either use multiple supply tanks in a “Cascade” type system or are direct filled from compression systems.