Welcome to the forum.
I'm going to cover points 2 and 3.
Point 2, I would say yes take a PLB (EPIRB). You can hire them, I think a search here on the forum will find where from.
Point 3, clothing.
September is still quite variable so you will need to be prepared for full-on winter walking, and also be prepared to alter your plans if there's a significant snow dump and you're not confident or experienced enough to deal with that.
Usually a 4 to 5 day weather forecast is possible, but they generally cover the major centres, none of which are applicable in the Tassie highlands. That won't cover your whole walk but give some indication of at least the first half.
I don't know about a ski jacket as such, I don't own one.
My layers for winter walking are (not being loyal to any brand and showing examples of similarity rather than actual) -
Up to 6 layers max, thermal, shirt, vest, thermal pullover, windstopper jacket, japara.
Up to 3 layers on legs, skinny legs so mot much to keep warm...
Thermal long sleeve top,
here is an example.
Long sleeve day walk top to go over that, perhaps something from
here. I use a polo type top, hard wearing and has lasted some years.
Then a vest over that, as is seen
here.
If it's really cold and I want many layers, I will put one of
these between my top and the vest. But I try and keep that layer dry because it is SO warm around camp and in the sleeping bag.
Over the vest goes something like one of
these. Basically, a warm layer like a thermal fleece jacket or pullover design. Some might replace that layer with a down jacket, maybe you can lose a layer with that but for a winter walk I would always have my 6 walking layers ready just in case, as well as my dry layers for camp / sleep of course.
Then a windstopper jacket, then the Japara. Most I've worn while walking is 6 layers and I needed them, but you do sweat a lot so putting the clothes on again the next day is not pleasant if they're still damp.
Legs - jocks, thermal long johns, walking trousers like the ones in the
photo, then waterproof trousers such as
these. Expensive overpants, but so worth it - I've been through many pairs of $35 rainbirds or 3 peaks, cheap and nasty, rip easy, fall apart, I have a box full of them. Should throw them out one day...
Dry clothes - thermal long johns, thermal fleece long pants, thermal long top, optional fleece top or use your vest / windstopper jacket / japara if they're dry. Gotta keep warm, lets the body rest properly and recover after a days walking.
15 degrees on the OLT in September? You'd have to be lucky to get it that warm IMO. Remember it's at altitude and for every roughly 100 metres above sea level you lose approx 1 degree.
I'll come back and address point 1 if no-one else does, but there have as you say been a number of posts about whether or not to take on a walk such as the OLT if you lack experience.
We're not here to turn you off, we want you to make the right decision for you, only you can do that after gaining advice and opinions from others, then weighing that up against your own ability, fitness, experience, preparedness, health, age, etc...