In the list of Personal Development Actions, there is an option to learn a Core Modifier:
"Learn A Core Modifier - You may only learn 1 Modifier per Event"
There is also the option to learn a Core Modifier from another person during an event, under "Learning from Others":
"You may learn any Modifier that is equal or less than twice your Engineering Theory Level by spending 5 minutes Roleplaying with a willing teacher."

Does the restriction "You may only learn 1 Modifier per Event" apply to only Personal Development Actions (in which case you could learn an unlimited number of Modifiers from other people at the Event, and at most one in the set of Personal Development Actions you undertake between that Event and the next, regardless of how many Personal Development Actions you have in that time), or to Personal Development Actions + at-Event learning (in which case you could learn at most one Modifier during the Event + associated Personal Development Actions, and the benefit of learning a Modifier at the Event instead of as a Personal Development Action would not be increasing the amount of Modifiers you could learn in total, but rather saving the Personal Development Action for something else)?

Thanks for the question!

The restriction applies to all sources of learning. You can learn a modifier at the event OR you can learn a modifier through a Personal Development Action. We very much encourage learning at the event, these can be fantastic sources of roleplay with your fellow players, and as a bonus, it saves your Personal Development action for something else, as you noted.

There are advanced classification perks that may let you learn modifiers faster though, if that is your goal!

A follow-up on the same system: under "Learning from Others" in the "Devices" chapter, it says:

"You may learn any Modifier..."

followed at the start of the next paragraph with:

"You may alternately learn a Blueprint..."

given that learning a Blueprint is presented as an alternative to learning a Modifier, do I understand correctly that you can do at most of the following per Event + associated Personal Development Actions: learn a Blueprint at an Event, learn a Modifier at an Event, or learn a Core Modifier as a Personal Development Action?

Yes, you understand that correctly, you can do one of those things.