I think it is regarded as transformerless in as much as it does not have iron core 50Hz transformers but instead the electrical isolation of provided by highly efficient HF ferrite transformers. I've not measured the frequency but like most modern SMPSU designs I would expect it to be at least 100kHz. I think all inverters must have some sort of transformer to provide isolation from the mains
An inverter per panel is approaching the ideal. I did seem one system where the main inverter communicates with a controller on each panel. Effectively it controls each panel to have the same impedance irrespective of the light level which might be different on each panel. In this way the maximum power can be extracted. I found this system after I had installed mine and sadly it is not retrofittable. I would have lost my original MCS certification and would have had to re apply and the tariff would have lower.
As you have a clear view of the sky at all times, then these clever schemes are not likely to benefit you.
Bob