Hello,
I am trying to achieve the effect of showing a camera’s FOV. I want to visualize this using a cone mesh that stretches from two selected points without the over scaling the mesh if point B is very far away. This effect must be able to run in the editor and in the Sequencer without clicking “Play”. Any help would be greatly appreciated. Thank you! 
Please check out the example I made below to see a visual representation of the effect.
If you make a blueprint that can perform the visual operation you can have Sequencer run the Construction Script of the blueprint in the editor. You will need to go to the Class Defaults and enable the option for allowing Sequencer to run Construction Script, and then ensure that inside your Sequencer you have the option enabled as well (it is on one of the dropdowns).
You will need to add your cone blueprint to the Sequence itself so that Sequencer knows to run the Construction Script on it when you scrub. You can optionally animate parameters on the blueprint (such as start/end location, color, etc.).
If you make fields to have objects A and B as parameters on the cone blueprint you can look up their locations in the construction script to orient/scale your cone mesh, plus provide limitations on length. You will need to call the same functions in Tick as the construction script is not run during play mode.