I apologise, KK, for my offensive wording there and I regret being so suspicious of your husband's motives. I should, perhaps, have clarified that my cynicism is born of personal experience ... and that abuse, financial or otherwise, can and does happen without the abuser's having sat down and formulated a deliberate plan. It's still abuse.
In remaining almost wilfully ignorant of your family's finances, I maintain that you value blind trust - and it is blind, since you don't have the facts and won't seek them - above fact. That is the mindset of an abuse target ... or a gambler, whose trust is (knowingly) abused by the gambling industry.
Sticking with gambling for a minute: Having interest-only mortgages on incestment properties is gambling. The property can only accrue equity if its value rises at a rate higher than the lender's rate. If you gambled on an over-geared market and the market slows, as now, it cannot supply this equity and you lose. Assuming you also renovate the properties to add value, you also gamble on your works adding more value than the cost of the works. If you borrow more to finance the works, you're gambling on quite a slim margin [added value - (loan capital + interest)] as well as on the market remaining buoyant enough to realise the margin.
So you are currently double-gambling on the property market and also gambling on your husband's honesty. Both look deeply unwise: The market's been advertising its disinclination to pay out for a good eight years now, and your H has advertised his dishonesty by taking money without telling you. Worst of all, you are gambling without knowing your odds because, as you say, you don't know the fact
It's bewildering that you don't know the facts and won't seek them! Why not? Why do you believe it would threaten your marriage to seek facts? Why doesn't that sound like an alarm bell to you?
Why aren't you audited? Who does your tax returns? Why won't you talk honestly with H's partner? I really feel you should ask yourself those questions, requiring a firmer answer from yourself than "I love him." You don't count love in banknotes, do you? So take your love as a given, and go ahead with the factual queries.
You can't "rein him in" without knowing what length of reins you need!