I understand I cannot legally require them to respond, unless the notice specifically requires it, like for instance, a notice of entry, but it's making it difficult to manage the property, and I'm not sure how to proceed.

What you want is an agent with experience who knows how to negotiate, watch your back, navigate difficult situations, understand contracts and law, provides an objective cross-check, etc.

If they are difficult, the current landlord may not give you a straight answer because they want them to find a place and move out.

CA is an expensive market and regarding tenant laws in my eyes more complicated/difficult as a rookie landlord.

It is rather difficult to make BRRRRs pencil out right now, so you should budget to leave some capital in the deal on the refinance. 

It almost feels like they make it difficult on purpose so they get extra revenue on late fees.

The interface is not very user friendly and things that should be obvious are difficult to find and take figuring out. 

. - cash flow is difficult now with long term rentals even if you pick markets that have lower purchase prices than Boston.

That alone makes it difficult to get the tenant to put utilities in their name.

As mentioned 90 day limit, which makes it difficult, and wuite pricy to get the STR certificate from the city.