What do you do when they threaten to leave? If you can't quite get to "let me help you pack" (that is expert-level DB) practice "OK" and leaving it at that. Then walk away.
For all the rest... practice some exit lines so you have them at the ready. Whether making a viable excuse ("I'm so sorry, I forgot something I need to take care of" and walking away, or being more direct "I'm really not interested in this conversation right now" or "OK, I hear you. Thanks for letting me know." I think there is a sticky with validation/exit lines that you might check out.
On my last thread, there is a lot of really good advice from AlisonUK and others about boundaries. How do you feel when he talks to you and you know he's lying or gaslighting you? Remove yourself from the situation. Protect your boundaries. You don't need to listen to something that is damaging to you. Alison had an incredibly helpful take on how she sets boundaries with friends/acquaintances, and how it works in her M with her H, that really helped me to better understand boundaries vs threats and how to enforce authentic boundaries. Boundaries aren't something you say. They are something you live.
And on gaslighting-- that only works if you listen and care what he says. Don't give him that power.
On the IC... do you work? if so, is there an employee assistance program through your job? Many employers have this and I think you normally get up to four free sessions with someone who could help you. It also works for family in case your H's job offers this. I'm wondering if this might be an easier step for you than setting up with an IC? Something that isn't permanent, is specifically set up to help you through difficult times, has a degree of anonymity that you wouldn't get with a friend or a long-term IC, and may have some simple suggestions for you or additional resources you can access?
you got this Blue!
Me (46) H (42) M:14 T:18, D9 & D11 4/19 - 12/19: series of escalating BDs 9/20 - present: R and piecing