First congratulations on the new addition to your family, just in time for the Holidays!!
Living in the White Mountains of New Hampshire has not made me an expert on cold weather diesel, but sure has provided opportunities to learn many lessons on how to get a cold diesel engine started in below zero weather.
For the problem you experienced, I think you had glow plug issues not a fuel gel issue. First, regular diesel will gel below -10F without winter blend additives. Winter blends reduce that freeze temp to between -20F and -40F at the pump (an area like NH -20F, Alaska -40F). Anytime you are in an area where the temps are going to drop below 0F, it is wise to add anti-gel when fueling just in case. The exact blend will of course change the gelling point, however in general, the base #2 Oil without any additives is good down to 0F. We use plenty of off-road #2 fuel in our construction equipment and below 0F is when fuel filters start clog with frozen particles from untreated fuel.
The Block heater is just that a block heater. This will not prevent the fuel from gelling in general although some limited benefit may be gained under the hood for the engine mounted fuel filter (which is why the primary fuel filter mounted under the truck gels/freezes first. Gelling takes place first in the fuel lines and more often becomes an issue in the fuel filter where the frozen waxes clog the filter. Most often in the primary filter (mounted under the truck rails) will be where the problem (clogged fuel filter) will be found. The fuel filters are preventing the frozen particle from reaching the injectors where real damage could be done.
If your fuel becomes gelled, it is almost impossible to remedy until the weather warms up without removing the fuel filter. At the school of hard knocks, I have more than once, found myself under a truck replacing a fuel filter to get a truck running. Also at the same school, I have been able to perfect my skills at replacing clogged filters on the coldest of mornings on a backhoe or two.
Now back to why I am wondering about your Glow Plugs. Generally, a Diesel engine should fire up with the assistance of glow plugs down to -10F. It will be rough and loud with lots of smoke but should start without much difficulty. Below that, very hard starting without a block heater, but is should be possible. Anything below 20F will start nicely with a block heater in use (for a 2-3 hours before starting). If you cannot start the truck above -10F without the block heater, I would expect one or more of you glow plugs are gone. From what you describe, you may want to get the glow plugs checked. Again, from experience, we run all our vehicles without block heaters down to 0F. We know if one of the engines has trouble starting at those temps that it is time to start diagnosing/replacing glow plugs. In fact, our 2006 PSD is starting to show the signs of glow plug failure it is will not start without a block heater at temps as high as 10-15F this winter.
Again, I am far from an expert and by no means a diesel mechanic, just a few decades of experience with the cold and Diesel engines to know,
(1) the modern diesel engine with glow plugs should start and run just find down of 0F without a block heater or any fuel additives
(2) if you need a block heater to start an engine above 0F, get your glow plugs and/or temp sensors checked/replaced.
(3) don't worry about fuel gelling above 0F, but if in danger of dropping below 0F, anti-gel additive is a great way to avoid having to crawl under the truck and replace a fuel filter to get your day started.
And finally, if you are going to be out of town, by all means add the anti-gel and plug in the block heater for your wife even if it is not suppose to be any colder than the teens... just make sure she sees the cord and cannot drive off with it connected... again
Hope this helps, Tim