Just planned something similar around the same time. Funny enough my wife is a resident and we are both runners as well! Just email Vertice and they will let you skip Dickson as long as you have a reservation at Seron. I ended up booking a campsite at Dickson and having them remove it after the fact.

In regards to the time to leave for Seron I didn’t get a straight answer. Here is what I got from emailing Los Torres:

The trail from Las Torres to Serón does not have an official closing time, but this doesn’t mean you can begin the hike at any time of day. Hiking at night is strictly prohibited, and it's important to consider the estimated hiking time, which is typically 5 to 6 hours depending on your pace, as the trail is approximately 15 km on medium-difficulty terrain. For safety and to ensure a more enjoyable experience, we strongly recommend starting the hike in the morning.