It does eliminate the need to add chlorine but the chlorinators (Pentair IntelliChlor) have a fairly short lifespan.  

The short answer is yes to MTR in multis, being respectful of the other tenants and the allotment of LTR, MTR, STR of the units.

Short Term Rentals.

There are cheaper markets that your current savings would go further and possibly provide better opportunity (almost definitely in the short term).

Luxury rentals, purchases and short term accommodations will be in high demand from Santa Barbara to San Diego and a drastic uptick in inventory for sale can be anticipated in the areas most effected by the fires. 

From what I’ve read in The Economist, short-term construction loans or hard money financing could be a great fit to cover the $90K needed.

The short answer is No, you don't need it.  

Ditto, I use PriceLabs for my STRs - here is a nice writeup ...Airbnb Smart Pricing Vs PriceLabs: An In-Depth Comparison For Hosts | Boost Your Short-Term Rentals

Currently, we manage approximately 40 short- and mid-term rentals, in addition to a few long-term properties.

In 2019, I launched my own revenue management agency to help Airbnb owners boost their income and achieve greater returns on their short-term rental properties.