Level 11

You're asking about interest but you're talking about principal.

Since 2019 is the conversion year the interest will need to be allocated appropriately BEFORE it can be limited.  There are several methods available to do this, however some of them require that the property be used for all 12 months.  Personally I use the daily average balance (which will reflect the actual interest paid for each bucket).  Often one of the other average methods will give you a better result but you can get into the weeds pretty quickly in the transition years.  I bill by the hour so it doesn't make sense for me to spend an extra 30 minutes of billable time down the rabbit hole in order to save a client $15 in tax.

There is a provision which allows you to "switch" your second home mid-year if you meet certain conditions.  One of the conditions is that you purchase a new second home during the year.  See Pub 936 under Qualified Home.

Main home is still main home with 100% acquisition debt interest.  The $150K "second home that's now a rental property" is good up until the earlier of 1) the purchase of the $300K "third home that qualifies as a second home after the purchase date" home or 2) the conversion of the "second home that's now a rental property" to a rental property.  Then you're over the acquisition debt limit and you're stuck calculating by how much for how long (which I assume changes every time there's a payment since most loan payments involve both principal and interest).

The interest on the rental property period for the "second home that's now a rental property" is calculated from the date the rental was available to be rented.  So interest after that date ends up on Sch E.

The interest on the home equity loan (assuming proper tracing of the loan proceeds) would be deductible on the Sch E for the property that was improved by the proceeds.  The fact that it was a home equity loan is not relevant for 2018 or 2019 (or likely through 2025 but that may depend on how much of TCJA get unraveled by Congress in the coming months).  If the equity debt limit is changed from $0 to something else (such as the old $100K) then the regs kick in and a portion of this becomes home equity debt with part of the interest deductible on Schedule A (unless you make a Sec. 1.163-10T(e) election to treat it as not secured by the home which kicks it all back to Schedule E).

Be sure you charge enough for this mess.