It reads to me like you're imagining a time where you're all under one roof but are not putting much consideration into the "getting to know you" process. You have only ever been in a relationship with Mrs Maple, and maybe you moved in together and got married fairly quickly. In order to find the next (yet concurrent) Mrs Maple you are likely to have to wade though a bunch of first, second, third dates. And if you get that far, then there's the months if not years of dating while living separately, since this woman will already have her own full life and all that comes with it. I'm honestly having difficulty seeing how this process of co-habiting as a pre-requisite for sex is going to be compatible with dating a grown ass adult. You're also putting the cart before the horse so much that of course Mrs Maple is freaking out. If she's threatening suicide, you need to STOP fantasizing about polyamory, get professional help like a doctor or psychiatrist, and then get a reality check about what it's actually like for married men to start dating even when their wives are fully on board, let alone poly-under-duress.
And if you ever do get to the point that there is another woman in your life, sometimes Mrs Maple might just being in the wrong and you shouldn't be taking her side.Its more like, she needs to know that I am there, I can step in and save her and back her up, but that I should also 100% take her side any time of any day because she is my wife. I do not, if she is wrong then she is wrong.