I’m as much as an Arri fan as anybody, but I can tell you first hand that the Monstro blows the Helium out of the water, and it is a serious contender against the Mini. Amazing color science, crazy resolution, and the low light is much friendlier than the Alexa.
You can use s35, just crop in. I shot a spot on s4s at 5k and it looked incredible. Full frame is becoming the norm, and as soon as that Alexa Mini LF comes out (I’m fully expecting an announcement this year), that norm will greatly accelerate, and you will find fewer places for your s35 lenses to fit.
They are out, just are way backordered. They are almost done getting out all of the units to their backers, and if you ordered now would more than likely ship out by end of June (all of this according to them of course).