I'm so sorry, DarthDad. I'm afraid there is pretty well a script you see over and over on MN when someone cheats, and ends the marriage as a result, and she is following it.
You need to focus above all else now on getting even a small one bed place where you can have DS overnight, and I think you need to as helpfully as possible suggest you have him from Tuesday after nursery to Thursday drop-off at nursery and every other weekend, to ensure she isn't doing all the grunt work of parenting - the tiring evenings and mornings as opposed to just weekends. Phrase it along those lines. It's true, after all. If she agrees and you establish that as the status quo, you then have two nights every week. You could also suggest you make it Fri pick up to Mon collection on the weekends you do have. That way, you have 5 nights one week and 2 the next, which is actually shared, 50/50 care, but stealthily asked for so she may not realise at first that that's what it in fact is. (But be prepared to negotiate over the Sunday night, especially - a good agreement is a lot more valuable than an angry one, because she's likelier to actually keep to it.) If she agrees now, when things aren't hostile, she will have a hard time arguing against it later - which is also why it needs to be via email.
In the same email I would explain that these days courts like people to use mediation instead, that the mediator listens to both and helps them work out an agreement over the kids that is fair to everyone, and then if both are happy, it gets stamped by a judge and becomes a court order. Would she be willing to attend a mediation session with that in mind, because you think certainty for everyone can only help DS, and help the two of you to move forward as friends and co-parents, blah blah blah? And that when the house is sold you can work out an agreement on the equity, hopefully with the same mediator. I would also take a look at the CSA website to see what you'd have to pay, and then offer a little more if you can possibly, by hook or by crook, afford that, straight away. It puts you in a strong moral position and unless and until in a financial order isn't something she can enforce - if she does go to the CSA they will actually award her less. And it will improve your son's quality of life.
She's been horrible, but the chances are as soon as money is involved she will start to be a lot less generous over contact with your son. Horribly, she is in a massively strong position there and negotiating contact patterns now, when she isn't anticipating you standing up to her over money, is likely to result in a lot better than you will get in a few months from now.
If you have paperwork that proves the loans were used to pay off credit cards in her name, and she's working, then ask the solicitor if they can be deducted from her share of the equity. Same with an income-proportionate (i.e if you earn double what she does, maybe suggest you should be paying 66%) of the bills and mortgage repayments from the point of separation.
I don't think you need to worry about a divorce as you're currently separated anyway and the only way to divorce is via either adultery or unreasonable behaviour, both of which require reasons and both of which will make her angry and hostile. The main thing is an agreed status quo of contact you can rely on later, preferably in the form of a contact schedule and shared residence order. (A sol will explain that that doesn't mean you have to have 50/50 care, it just means the courts recognise the child has two homes.)
Sorry to sound cold and pragmatic. You are in hell and I am so deeply sorry. Her behaviour is horrible. But the priority obviously needs to be protecting your relationship with your son, and then your financial interests, and you are so right in saying you need to play the long game in order to achieve both.