Sorry about my crappy timing.
The "6 years prior to TFA" isn't necessarily when everything went down, but it's the *earliest* that those events you listed could've happened (if you take novels as canon). And with respect to connecting with The Mandalorian, the FO had not emerged in public prior to that point - which is about 20 years after the current Mando timeline.
It's when Leia decided to start investigating potentially nefarious activity involving shady underworld dealings. When she realized the scope of what was actually going on (that it had much broader implications than just an underground crime syndicate), it eventually led her to form the Resistance.
At that same time, Ben was still Luke's student and Han was still with Leia. But we don't know exactly how long thereafter Ben slaughtered the other students. What is certain is that it could *not* be more than six years prior to TFA, but could be as few as 2-3 years. Maybe the comics get more specific than the novels, but I didn't read many of the comics.
The key takeaway from the novels is that the New Republic Senate had FO plants working there covertly to cover up their existence and expansion. Leia got too close to discovering their plot and was disgraced out of her position in the Senate for it (to limit her influence there). And this was the turning point in her being forced to splinter off and investigate (and ultimately combat) Snoke and the FO in a non-government capacity.
Even to someone as in-the-loop with the New Republic hierarchy as Leia was, the FO was completely off the radar and not known to exist until that point. So, during the Mando timeline (20-25 years prior to TFA), any exposure of the FO would have to be compatible with being outside the notice of the New Republic. Otherwise it'll be yet another significant retcon.