How to create a calendar event but keep my external calendar default alert times?
I had the same problem. I use Calendar all the time for email alerts and this has become very tedious. 

Here's a workaround:

Make a Calendar entry as a template: 

Create a generic email alert for one day ahead. Use a title you will remember like "one day alert," or maybe "1D" for a short name.

Use the template for a new Calendar entry:


If it's not immediately visible, make a same-date copy of the alert (I use option-click-drag). 
Retitle as desired &amp; add any needed details. 
Change the date (the date the template resides in) to the date you want. Doing this last will avoid an additional email alert.


Hope that helps.

Ps. You have to do this for each different lead-time you want for an alert. I nearly always use one or two days, so I created a template for each. I also made one that does both a one day and a two day alert.