This is especially concerning since, once a certain number of emails per month is exceeded, additional charges start to accrue.

I’m starting to save for my first rental property and I’m wondering what type of account I should store my money and as I am accruing it to gain interest.

If not, I don't see why it matters that she accrues a monthly late fee.

If the accrued interest amount is over $15K for 2019, your ex-in-laws would technically be required to file a gift tax return for the "gifted" interest amount.

We are mostly concerned about the short Term, point 3B, and I have more of the contract which I will post here underneath:ClaimabilityThe amount owed (with accrued interest on the entire principal during the entire term, three months extra interest and the costs) will be immediately due and without notice, notice of default, written warning or other formality due in the following cases:a. if the debtor may be negligent in complying with one or more provisions made by this deed;b. if the debtor has been granted suspension of payment or the debtor has been declared bankrupt, the debtor's property has been wholly or partially seized by third parties and if the debtor otherwise loses the free disposal of his assets;c. upon seizure of the collateral;d. upon alienation (whether or not only economically) or expropriation of the collateral in whole or in part;e. in the event of fire damage or any other depreciation or potential depreciation of the collateral, whether or not due to the application of any government measure to the collateral; f. if another debt for which the collateral may be linked to a mortgage becomes due and payable;g. when (one of) the debtor (s) dies.

For example in my state (PA), a judgment lien can accrue up to 6% interest.

My idea was if we ended up needing to go to court at least the rent that was accruing with now be at a much higher rate. 

The lien is "upon any and all actions, claims, counterclaims and demands accruing to said injured person or accruing to the legal representative of said injured person [in case they die], and upon all judgments, settlements and settlement agreements entered into by virtue thereof on account of injuries given rise to such actions, claims, counterclaims, demands, judgments, settlements, or settlement agreements which necessitated such hospital care."

And if that happens 4+ years down the road we will still come out ahead even if we take a bit of a loss on the resale due to the accrued cash flow. 

You do not have rights to the home equity that may have accrued, the borrower gets that.