This is a fairly common arrangement and can be acceptable if properly designed. Some considerations:

1. The main concern is low temperature downstream of the mixing point, in case of mixing valve failure. Perform a LOPA (layer of protection analysis) on the safeguards. A shutdown table showing the action of each shutdown would help with understanding the layers of protection. The primary means of temperature control could fail if the temperature control valve fails open or if the sensing circuit fails high. The safeguards can then be evaluated to protect downstream equipment from getting too cold. The layers of protection should include two independent means of low temperature control: detection and shutdown. For example, the safeguards could consist of both stopping the pump and shutting an emergency AOV. 
2. Another low-temperature concern arises when the storage vessel AOVs are UPSTREAM of the normal temperature element (TE) to detect low temperature gas past the vaporizer. This is less of a concern for an ambient vaporizer since they can't fail "suddenly", but still it's best practice to install a TE upstream of any equipment that could experience brittle failure. It's possible that the buffer vessels will be stainless steel (or other material capable of low temps), but they are most likely carbon steel with a lower temperature limit of -20 deg F° or -40 deg F°. In this case, it is recommended that a TE be installed upstream of this connection. The temperature setpoint should be verified to be no lower than the rating of the vessel materials. It might also be possible (but unlikely) to backflow from the cold line if there is a leak on the storage vessels. The TE might protect against backflow depending on its operation. 
3. It might be possible to block in the vaporizers from the relief valve downstream. Verify that there is a relief valve that protects the vaporizer from a blocked-in liquid situation between shutoff valves and/or check valves. 
4. There could be sections of line on the vaporizer bypass where cold gas/liquid could be trapped between valves if closed when cold. For example, the double block and bleed valves. Verify that these lines can't be overpressurized in a trapped cold gas/liquid scenario. Install a relief valve(s) where needed or remove some of the valves to reduce the possibility of trapped product.
5. Failure of controls or control valves could "deadhead" the pump, leading to trapped liquid and/or an operating pump being blocked in. This could result in a relief valve lifting and/or an overpressure situation if no relief valves are present. Verify the intent of the AOV operation when the pump is running, verify that there are sufficient relief valves on the pump discharge, and determine if there is a blocked in or deadhead situation. 
6. Even if there is all stainless steel construction downstream, the dispensing hose (-40 deg F°) or the vehicle itself (-40 deg F°) still must be protected from cold temperatures.

The information provided is not only outdated in terminology, but also misleading in quantifying the dispersion of hydrogen in terms of comparison of diffusion of air in air. Hydrogen diffuses 4X faster than air, and the rate of mixing has many variables, so there isn’t just one answer. However, it’s generically safe to say that an initial hydrogen gas cloud outdoors and unconfined will dissipate below the flammable range very quickly, likely in just a few seconds once H2 flow is stopped. The factor of 4 mentioned in the question refers to a molecular (laminar) diffusivity, but almost all practical dispersion scenarios are governed by turbulent diffusivities. The turbulence can either be atmospheric (ambient) turbulence or the turbulence associated with the hydrogen release itself, i.e., jet turbulence or vessel failure induced turbulence. Regarding the rise time of hydrogen clouds, these can be calculated from equations in the SFPE Handbook for the two cases of an instantaneous release and a steady-state release. The cloud liftoff time, tl, for an instantaneous release is given by the following simple empirical equation from Beyler’s chapter Fire Hazard Calculations for Large Open Hydrocarbon Fires because it is just based on buoyant rise observations: tl = 1.1m1/6, where m is in kilograms. Therefore, a 10 kg release would lift off the ground in (1.1)(10)0.167 = 1.6 seconds. The initial rise velocity of the buoyant cloud is proportional to "gD(28.8  2)/28.8"1/2, where g is gravitational acceleration, D is the cloud initial diameter, and the 28.8 and 2 are molecular weights of air and hydrogen, neglecting any initial entrainment of air into the cloud at cloud formation. Thus, a 10 ft diameter cloud would begin rising at a velocity on the order of 17 ft/s, and a 20 ft diameter cloud would have an initial rise velocity of about 25 ft/s. In the case of continuous releases, the rise time, tR, of a buoyant plume front from a suddenly initiated release can be calculated from a combination of equations 61, 62, and 16 in Heskestad’s chapter Fire Plumes, Flame Height, and Air Entrainment, where z is the rise elevation of interest, u is the hydrogen release velocity, b is the initial plume (cloud) diameter, and the densities are 2 and 28.8 as above. A consistent set of units is needed for z, u, g, and b to do calculations with this equation, which is based on a combination of theory and experiment.