Temporal vs. Spatial Summation 

Just say:Concept could be easily understood than my comment

Thank you! I'm glad you liked the video!

Thanks for the encouragement, I'm so glad it helped!

Don't quote me because I may not be correct about this, but I believe that is due to the relatively high charge at the synapse diffusing to all of the rest of the cell body. For example, say you were to put a drop of food coloring into a small cup of water. There will be a high concentration of that food coloring at the site that it enters the water, but it will quickly distribute throughout the rest of the cup and become less concentrated. I assume this is a similar process to what happens to the charge from positive ions entering the neuron at the synapse as they progress toward the axon hillock.

Glutamate typically acts at excitatory receptors such as NMDA and AMPA receptors. These are very complex and can promote the passage of Na+, Ca++, and K+ through the membrane depending on the local voltage. You are correct though that K+ flows out of the cell and decreases resting potential. That probably should have said Na+ and Ca++ for EPSPs instead.

While that's true to some extent, Na+ inflow is far greater than K+ outflow resulting in a net increase in membrane potential

You are right, it will be calcium instead of kalium