It wouldn't be Melee but still Lightsabre. The Lightsaber skill encompass all types of Lightsabers and any kind of variant thereof. It's also to your advantage to use Lightsaber skill. In addition to opening up the possibility of not relying on Brawn as your attribute through the 6 fighting forms but it also means that you're only raising one skill as opposed to 2 to be competent with a single weapon. After all if your rifle is sundered and you pick up the lightsaber bayonet to use if it's tied to Melee you are no longer able to use that lightsaber as well as you were before. So in order to remain competent you'd need to raise Melee, Heavy Blasters and Lightsaber. It works out better just to have to raise 2 as opposed to 3 skills.
Of course it would be to his advantage to use the Lightsaber skill, for all of the reasons you listed above, but that doesn't mean it fits. The relevant skill is not determined by the primary striking surface of the weapon. A number of Brawl weapons are blades or even blasters, but they use that skill because they complement unarmed fighting techniques and leave the hands free to grapple. Ranged - Light is used for one-handed weapons, whether fired or thrown, while Ranged - Heavy covers two-handed weapons and Gunnery is for heavy weapons that are operated rather than wielded; none of these distinguish between the type of projectile involved.
Every single published weapon that is used with the lightsaber skill is a light, streamlined weapon with minimal physical components and a massless striking surface. Even the lightsaber pike, the only lightsaber with a physical haft of any significant length, is made of super-metals that are both lightsaber-resistant and incredibly light; other than its length it shares none of the vulnerabilities or potential drawbacks of conventional polearms. All of these features mean that the weapons that currently use the Lightsaber skill have distinct handling characteristics unique to their grouping, namely the ability to be wielded with an incredible amount of manual dexterity regardless of the physical strength of the user. They also mean that the lightsaber can be used in ways that other weapons simply cannot.
A lightsaber bayonet, while attached to a ranged weapon , would share few of the handling characteristics of a lightsaber. The physical body of the rifle has much more mass than any lightsaber hilt, and the inertia of that alone would impair the handling of the weapon as a whole. Furthermore, that mass would not be distributed in the symmetrical manner of a lightsaber pike or even a conventional polearm, as the various components of the weapon are attached in different directions at different angles, unbalancing the weapon and offering a number of protrusions to catch up the wielder. There is also the issue of the construction of the rifle itself; the components that make up the majority of the rifle-bayonet combination are not exactly intended to be part of an impact weapon and are certainly not capable of resisting blaster fire or weapon strikes like a lightsaber is. All of these aspects together suggest that in terms of handling characteristics and fighting styles, a mounted lightsaber-bayonet has almost nothing in common with a traditionally-wielded lightsaber.
Finally, there is the issue of the various forms of lightsaber combat. These techniques were developed specifically for use with traditional lightsabers to take advantage of their unique characteristics in comparison with conventional melee weapons, and are practiced in an almost ritualized way by the few lightsaber-wielders in the galaxy. As I have demonstrated above, a lightsaber-bayonet has much more with any number of metal-bladed weapons that make use of the Melee skill than a lightsaber.
Now, everything I have said only applies to a shoto while it is mounted as a bayonet . Once removed, it could be wielded like any other lightsaber, and would certainly use Lightsaber skill and the various lightsaber forms as normal.
Edited by Joker Two