My realtor tells me its invalid because it was sent prior to midnight when the notice to perform would expire.

Perhaps you can procure new tenants as your existing leases expire, and get there quicker. 

Since you are purchasing that property I will point out that if the tenants have existing leases with a set term (meaning they aren't month-to-month) you cannot switch responsibility for utilities until the current lease is expired and a new one is entered into. 

Assuming the usual rules-of-thumb for financing leaseholds (loan term less than lease term, for example)is it equally as difficult as financing a property with a lease expiration (just) under 30 years, which banks are very leery to do?

Now, the leases are about to expire and the tenants would like to stay. 

Here in the bay area I believe the expiration date is June 30. 

Your contract called for a contingency period which had expired prior to your request for cancellation. 

Tenant and Landlord hereby agree to continue this lease beyond the expiration date on a month-to-month basis.

If they currently have a lease in place, you would normally need their lease to expire before increasing their rent. 

If they don't pay in full by the time that expires, I file for eviction with the court.Do you know how many times I talk to the tenant during that process?