NPS and guiding services are always speaking to what I've come to call the 'lowest common denominator'. I don't like using a dismissive term like that, but they have no idea who they are talking to have have to give a bit of the 'worst case' scenario talk pretty much all the time. Not to say we all shouldn't pay them heed, its just taken with a grain of salt, depending on your experience and know-how.
You can still have significant storms on Rainier at any time of year. Due to this and the 'remoteness' of this route compared to the 'walk-up' routes, namely Dissapointment Cleave (DC) and Emmons Glacier route via Camp Sherman, where there is a constant flow of folks that could potentially help, I would personally want to have a fairly robust shelter if some weather came in. I had my Bibler Torre on Lib. Ridge. It was overkill for the weather we ended up happening, but if someone got hurt, sick, weather came in etc., we felt the need for a secure shelter. The 4 folks that died the week before we did it had lost their tent and succumbed because of that.
Bringing one tool and one axe ('technical axe', as you said, should have a properly designed pick so that is will work OK for a short section of hard ice--depending on conditions, however, most of the route could be ice on 40 deg + slopes) is very common and probably the right choice on this route.
I think 30 deg would be OK, if you have (which you will) insulating layers you could use in conjunction with the bag if it got colder. A 20F bag might give you just a bit more insurance, however, but 30F is mostly likely OK.