I have to admit I'm curious. Your update says that your husband was fluctuating daily on how he felt about you and Liam. Obviously there was not, let's take a break decided on and that's cool, up to you guys how to proceed. However, I'm curious, was anything ever discussed? You know, how you wanted to proceed, what being poly meant to you, how a poly relationship might look like now, in the future? Or was it just "I want to hang with this guy and it's just friends, so ignore the make outs previously and I'll give you time to get used to it."
Just because honestly I have never (I know, I know, never say never) seen letting a monogamous person have time to 'come to terms with it' work. Moving from a mono relationship to a poly one or a mono/poly one is work for EVERYONE.
Having spent much time on lists and groups specifically for mono/poly one of the things we talk about is how there is no move from mono to poly. It's at best, mono, ethically non mono, poly. Sadly a lot of times though it's mono, cheating, non mono, ethical non mono, poly. I'm not saying you are one or the other since you two would know better, I'm just saying leaping straight from mono to poly doesn't work. There needs to be time where ALL PARTIES are working on what they feel, what they want, and how to make adjustments. I'm curious as to if any of that was done.
Also, curious as to why you would jump to the conclusion that your hubby went behind your back. If this relationship started as 'just friends' but there were make outs and kissing that 'just happened' and then back off to just friends again. If Liam knew you were married and if none of the above discussions went on with hubby OR Liam, it's totally possible that his feelings that something wasn't working were truly his own.
TL;DR - discussions need to happen with everyone and jumping to conclusions of sabotage is not good for ANY relationship.
Me: Late 30s pansexual poly.
DH: My husband of 19 yrs and father of 3 teen girls.
DC: LDR of +4 year