The Daily Mail had an article saying they first did a podcast together in Dec 2017.
From the timeline I have:
Dec 2017 – Lori and Chad made their first appearance together on a podcast (Daily Mail)
I can't recall if we have any other verification of that claim and Daily Mail can sometimes get things wrong... I'll see if I can find the article I pulled that factoid from.
I checked all the podcast listings for the
Voldemort 
podcast group in an effort to see when they could have met through that avenue.
Preparing A People Podcast Network | Himalaya
Per DailyMail, yes, that's incorrect. There is no podcast listed in December 2017 that features Chad or Lori, and definitely not together.
I see near the bottom (earliest podcast on the list Episode 001 on July 12, 2017 of the titular podcasts) the owners and co-host CD is introduced. Chad is always included in the episodes as a Co-Host or with his full name.
Every time hosts or speakers that are part of the podcast are mentioned in the synopsis, they are by first name except their introductory podcast. LV is introduced as Lori Vallow for the first "Time to Warrior Up Podcast - Introduction" podcast on Dec 4, 2018. All later times she is mentioned as Lori.
I think it is likely that the two met at some point before December 2018 and through interactions with the podcast group. There is a possibility that they met in person when Chad was in Mesa, AZ on July 13, 2018 for a conference (I've shared this info earlier:
ID - ID - Joshua Vallow, 7, & Tylee Ryan, 17, Rexburg, Sept 2019 *mom, arrested* #18). He was also in Mesa for the November 2018 Event so there's a possibility that was when they met. Here he is in the audience in white shirt and tie.
(source: himalaya.com url above).
Do I know exactly when he and Lori met? No. But it was definitely before Tammy died in October 2019, Chad said as much to the landlord in November 7 conversation in Kauai. It's possible to have been before July 2018. It's possible to have been at the "Zions Camp" in 2017 (The podcasts about the camp started in August, camp shortly before). There were reports from sundry sources that Chad and Tammy were at a camp in July 2017 (before Aug 21, 2017 eclipse) and Lori was there.
Your awesome timeline confirms that:
"
June 2017 – Family members say Lori and Chad first met sometime around June 2017 (Daily Mail).
Summer of 2017 – Chad meets author and podcaster, MG, for the first time after he speaks at a campout in Northern Utah. MG invites him to speak in her hometown of Mesa, Arizona (Chad's Forward for MG's book)."
If MG invited him to the speaking event in Mesa, my post linked above shows the connection there in July 13, 2018. I'm not sure about previous dates. We know that Lori became involved with the podcasting group with MG and JasonM.