It’s whatever you agree on, but I’d be ready for things or circumstances to change in the seller’s mind over that long of a period, especially it the property appreciates significantly above your price.

The expected time from closing on the purchase to closing on the sale is six months, perhaps 12 months in extenuating circumstances. 

It depends on her goals and facts and circumstances.

Then, take your original $100k (you never spent it...no matter how hard you tried), and keep it.Never under any circumstances, for any reason, spend your seed money. 

I wonder what the circumstances were that warranted that kind of drop in price to something that isn't even close to market value.

I’m thinking you’d need some other “circumstance” to quickly sell a property “bought for long term investment”, under a 1031.

Outline what happens to partner's interest in circumstances of death or divorce.  

You should consult a local attorney familiar with laws  and processes of your jurisdiction and the details of your situation which may significantly affect circumstances and outcomes.

That way I know I can cover most unforeseen circumstances

It all comes down to your financial circumstances and how much you need the cash.