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The 50% rule is valid, and used by experienced investors for deal analysis because it works, and is surprisingly accurate across a wide spectrum of properties: https://www.biggerpockets.com/...It's common for listing agents to paint a rosy picture of a property's performance. 

While you can certainly send tenants an account statement, or notice showing what they owe, be very cautious about sending a pay or quit notice (we commonly call it a 3-day notice here in Florida), or any type of notice that mentions or threatens eviction. 

If it says they cannot leave things in the common areas, do it.  

They will definitely have one for July 4th and they have them very commonly.

We had one in our SFR for 12 years & reglazing was a common reoccurrence.

We don't yet have the density issues that other cities like Portland (where ADU's are quite common) have.  

I don't think it's too common to have first and last months rent as security deposit.

It all depends on what you consider "a good deal" multifamily is fairly common in the San Gabriel Valley, and while the properties tend to be old, many are fairly well maintained.

An appraisal contingency is common; no one will balk at it but in a seller's market, the seller can just wait for the next buyer, a different appraisal, possibly a different outcome - rather than negotiate with you.