Tenant Breaking Lease

in California

Tenant(1) wants to move out before end of lease. moved in April 2014 for a 1 year lease.
Tenant(1) is nice enough to find someone (Tenant(2)) to replace her.

Question 1: do I hold onto the SD until Tenant(1) lease expires. isn’t Tenant(1) still obligated until 1 year expires, what if Tenant(2) doesn’t work out, is Tenant(1) considered like an guarantor for Tenant(2)?
I fully understand that I cannot charge 2 separate tenants for the same place. but this may be different for the SD obligation. ( I would send Tenant(1)a SD statement within 21 days showing why the SD is still being held by LL)
Question 2: Tenant(1) submitted a notice to vacate. how do I answer it without waiving my rights as a LL?
Do I write ” your notice to vacate is conditionally accepted as follows: A) Tenant(2) signs lease and successfully pays through March 2015. B) …….

FYI. a walkthrough will be conducted in between the 2 occupancies.
at this point Tenant(1) is expecting to pay through Nov 7 and she thinks Tenant(2) would be able to move in the next day. While I would like to accommodate that; this is unrealistic, as I do expect that I will need to prepare the unit for the new occupant.

So I wrote back this to the tenant.
I will get back to you
I will be meeting with the applicant on Tuesday.
Please let me know when I can schedule the walk through when you are present.
At this point , you are still obligated to pay for the entire month of November. Upon a successful transition, you will be owed a refund.
Because if the applicant changes their mind, your obligation is not satisfied.