Which of these interpretations of the definition of a cone's area of effect is the correct one to base my template design and construction on?
Answer: Option 1
You say:
The problem is that the rules for the shape of a cone AoE aren't very precise, and when you start looking at them closely, there are a bunch of inconsistencies.
and
There are three incompatible interpretations of the maximum length of a cone
I disagree. It is clear what precise shape they describe, and it is the simplest way to describe and use a cone area of effect in this game, particularly given (but not limited to) the game supports the use of a grid and miniatures. It is quick to use, easy to understand, does not require a calculator or trigonometry, is difficult to argue about and you can draw it easily on any scale.
The rules as written:
Areas of effect (PHB p.204)
A spell’s effect expands in straight lines from the point of origin.
Cone (PHB p.204)
A cone extends in a direction you choose from its point of origin. A cone’s width at a given point along its length is equal to that point’s distance from the point of origin. A cone’s area of effect specifies its maximum length.
There is also a picture on that page showing the shape produced.
Areas of effect (DMG p.251)
Choose an intersection of squares or hexes as the point of origin of an area of effect, then follow its rules as normal.
Explaining how these rules work together, the process to follow when defining a cone area of effect is:
Choose an intersection of squares/hexes as the point of origin - i.e. a corner of a square, often the one the caster is standing in;
Draw a straight line away from the origin of the effect to the range of the spell as stated in the description. This is always in multiples of 5', the size of a standard square on a D&D map, in your example 30', which is 6 standard sized squares/hexes on a grid;
The width, or more accurately diameter, of the cone at any point along the line above is equal to the distance from the point of effect at that point. As an example this means that at 10', or 2 standard squares, along the line from the origin the cone is 10', or 2 standard squares, wide.
This implicitly defines a flat bottomed cone, as shown in the diagram on p.204 (though you have to be careful with illustrations, in this case it is accurate). It has a width at it's furthest extent along the straight line equal to the length of that line. A 30' cone will extend 6 squares from its origin and be 6 squares wide at it's fullest extent. All straight lines, no curves.
This precisely describes option 1.
Option 2 and option 3 are definitely not what the RAW describe.
As to encouraging players to cast "off centre", yes of course. A player will probably set the orientation of an area of effect to maximise the number of targets whatever the rules are. They will do it to reduce the amount of unwanted collateral damage as well.
As a related aside, it is important to remember that a grid (if used) is a meta-game guide and does not exist for the characters in the game world in any way, so there is no "correct" orientation for them, just the one that reaches/effects the desired target(s).
DM judgement is required when the cone is not cast parallel to the ground or if the ground is not flat. The DM will have to adjudge what is effected but I strongly suggest you avoid too much discussion and definitely avoid calculators and trigonometry. It should be done as much as possible "by eye" and what is fun and good for the game rather than what is exactly precise. The DM should use the following rule to decide if any parts of the area of effect are blocked:
Areas of effect (PHB p.204)
A spell’s effect expands in straight lines from the point of origin. If no unblocked straight line extends from the point of origin to a location within the area of effect, that location isn’t included in the spell’s area. To block one of these imaginary lines, an obstruction must provide total cover
A spell's description fully describes its area of effect in three dimensions, so when any dimension is omitted for a non-flat spell effect, that's because it's unnecessary by natural reading and symmetry. So since it's not described as a ribbon or a wall, and since the bolt's width is given as 5 feet, that's the width radially in all directions around the centreline of the line — horizontally, vertically, and diagonally.
When parsing spell descriptions, remember that D&D 5e is written in natural language and doesn't assume a grid (which is an optional rule), so spell descriptions are written assuming the reader will simply picture the most logical natural shape and effects that fit the description, such as “width” being sufficient to describe the other two dimensions of a long thing that isn't already describe as flat or tall.
Best Answer
The rules you just quoted say yes, so barring a DM decision otherwise, a beholder can effectively antimagic itself at will. Granted antimagic only suppresses magical effects, it does not remove them. So while the beholder can "turn off" your blindness/deafness, it will have to shut down any spellcasting or other magic of its own in order to do so, and in order to resume casting spells it would have to let your blindness spell come back into effect in order to do so.