I add this here to help others too: Bear in mind that if you'd like to flash a zip file on a secondary Rom, you cannot do that with the TWRP_multirom_hammerhead_20160210.img. You need the older version TWRP_multirom_hammerhead_20150630.img for that. Which is the best SFOS version? I think it depends on what you would like to use it for. If you are happy with the native browser and apps and don't need AD, I think always the latest version is the best. However, if you need AD, you need to stick with an older SFOS. (Although, because of the old AD, in there, many apps do not work properly or do not even start.) Camera? Video recording is generally broken on this base (CM12.1), so that option is not available. (In addition: on SFOS 3.3 the viewfinder displays a black screen when you take photos, this is fixed in 3.4 and 4.0) Whatsapp will only work for texting and calling but won't work with video at all. I cannot comment on how to install a newer android as a secondary Rom as I never needed it. You need to find a Rom that works and you trust and try it. Then let us know you experience.