OH NO FURNACE INSTALLED INCORRECTLY BY SELLER! What do we do?!?

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Happy Holidays Everyone! 

I would appreciate any help or guidance on this.  We just bought a house located in East Bay SF - California. Escrow closed December 2, 2020.

Background: Prior to purchasing we had paid for a home inspection done by home guard and the inspector noted:

"The heating systems energy source was shut off and the system was not operational at the time of our inspection. We cannot offer opinions about system performance or general state or repair. We recommend inspection by a licensed HVAC contractor when service has been restored. (See Photo 9)"

So after that we had submitted through our realtor a request for repair because the seller would not give us a credit. Our Realtor had requested  "2.) HEATING: pg 5 (#9) Gas heater needs to be turned on and rechecked."  

Seller said they fixed it. Also, we paid the asking price because the seller realtor was firm on how good the work was done on the house and they had pulled permits.  Later, I go up in the attic and notice there is a 1/2" of water collecting in the drain pan underneath the furnace. I have no clue about furnaces but I know standing water is not good. I tell our realtor and also, have my partner's dad check it out (he's knows hvac) and he tells me this is a problem.  It also had rained and where the vent pipe was installed caused a leak in the roof. 

Long story short - the seller won't provide the contractor/person who installed it. What he did was provide another HVAC company name. Our realtor talked to the company and they came out and looked at it. They said that it was missing a hose, there was no slope, that the drain pain was too small and there should be two drain lines and not one.  They have fixed most of what the first person did but they had to order a part that was missing so we are waiting for that. 

Now comes the big question ....what is our recourse actions if the seller doesn't want to pay?

Other side note, the seller paid for home warranty - I submitted a service request online to look at the furnace (paid $75). The technician came out and empited the drain pan and said there was a leak that he supposedly fixed. Did not mention anything about it being installed improperly, that it was missing equipment, etc. 

Thanks all for your help this is our first home purchase so any advice will help!!!

Marie