With two certified inverter brands, ZCell has as many compatible...

With two certified inverter brands, ZCell has as many compatible inverters as Tesla's Powerwall. Unlike the powerwall though, there is the intention of more inverters to come.

Note that Tesla only recently committed to a second inverter brand, SMA:
http://www.greentechmedia.com/articles/read/tesla-confirms-detail-of-powerwall-improvements

note how in the article they also acknowledge noise issues with the first generation Powerwalls.

how many inverters are certified and how many installers are trained is only really relevant once ZCell is fully launched. Why would Redflow rush inverter testing when it still has time prior to launch to ensure it is able to give as much feedback to the inverter manufacturers as possible with the potential for firmware improvements etc.?

There were complaints of how few installers appeared to be trained and one photo of Simon training 40 installers at a time was enough to lay those concerns to rest. I suspect it will be the same once Redflow notifies us of the next certified inverter brand(s).

Again, the ZCell is designed to work with most inverters, not just certified ones. When an inverter is certified, this means that the full functionality of the battery and inverter are enabled and have been tested using a digital link (e.g. inverter identifies zcell, zcell communiates state of charge to the inverter) and from what I understand with the relevant settings programmed in to the inverter (no manual fiddling). Alternatively, there are two other compatibility modes that the battery can connect to the inverter with and ways for the battery to pretend to be a lithium or lead acid battery.

Clearly using a certified inverter is preferable but any existing inverter should work, albeit with manual configuration of the inverter and possibly the state of charge not being displayed accurately especially with older inverters designed solely for lead acid batteries, which have a different voltage profile.