That being said, I don't think the weapon summoning is specifically necessary, mostly because it becomes redundant with pact of the blade. The spell list and the abilities push you towards pact of the blade, much like Hexblade, but it isn't necessarily required.
I agree with this.
The weapon summoning is mostly thematic, as when a magical girl transforms in the genre she usually also creates a iconic weapon. It's why they only have proficiency with martial weapons and shields when transformed, and no longer have the proficiency when no longer transformed. It can be fulfilled with pact of blade, but I didn't want the Bestower Warlock to have to wait until level 3. My train of thought was that a Hexblade gets 24/7 medium armor and martial weapon proficiency, which wouldn't work with what I was going for with this class.
1) Lower the AC bonus to 16 or 17 to start. There's a reason that full plate (which is AC 18, with no Dex requirement) costs 1,500 gp; it's because PCs shouldn't have it at 1st level. We don't have a PHB example of a magical ability that gives you a heavy armor equivalent AC calculation, so I would look at the text in the equipment section for heavy armor for the best way to word it. Emphasize that it specifically doesn't gain AC from a higher Dex, it doesn't have a Strength requirement, and that it stacks with shields, but no other armor. (And don't use barkskin as a model, that spell has some of of the worst wording in the entire PHB.)
I'll do this.
The reason why I made the AC 18 was because I was scaling it with the future potential of medium armor. I assume most Hexblade's will eventually get +3 Half Plate armor end game, bringing their AC to 20, but you're correct that I didn't take into account that Hexblade's need a 14 in Dexterity to pull that off, while the Bestower doesn't. I'll probably make the starting AC 16, and have it increase and level 9 and 17 as normal, bringing for a final AC of 18(which can still be increased by shields or abilities that give +1 AC like the Warforged UA race)
2) The +Cha to attack ability is probably overly broad for level 1. Remember, Hexblade only allows it for 1-handed weapons until you hit level 3 and take blade pact. The current conception would allow you to take a one level dip, and then do a bard build with +Cha to bow attacks, Elven Accuracy, Sharpshooter, and Swift Quiver, which would be amazing.
While this is true, it should be noted that the transformation does take an action to use. With the exception of a fighters action surge, the character would have to give up the first round of combat to transform instead of dealing damage, and combat typically doesn't go longer than 3-4 rounds.
Also, I'm toying with the idea of making the transformation twice per short rest, but instead of lasting an hour, it lasts a number of minutes equal to Warlock level. Do you think this might be more overpowered than the once per short rest but 1 hour?
3) Remember that giving out armor with no Dex requirement means this character can dump both Str and Dex (normally not doable if you want a decent AC.) That's quite strong in a point buy environment, and means the other abilities need to be considered in that context.
This is true while they are transformed, but they'll spend more time not transformed than transformed in the game. It's once per short rest and lasts an hour, and assuming there is an abundance of conflict in a day, there will be fights where they'll need dexterity for their AC, or need the extra boost in saving throws. They have no proficiency in martial weapons or shields when not transformed, and don't get medium armor proficiency like Hexblades.
4) Remember, you can always add higher level powers in the form of new, patron-specific, invocations. Not everything needs to fit into the 1-6-10-14 ability paradigm.
This is a good point. Which ability would you recommend making an invocation? I'm leaning towards the end transformation avoid death with 1 hit point ability.