Hello there! Here are my answers, though you ought to take them with a grain of salt as I'm a data point of one, and my answers will definitely be prejudiced by our experience.

I purchased up front for two reasons: a) I was purchasing the three-for-two tickets, and was unsure if that offer is even available in person (anyone who knows more please chip in) and b) though I learned somewhere that up to a third of the tickets are available in person on the day or up to a couple of days (?) in advance, I was unwilling to take the chance that they might be unavailable when we got there.

That said, we were able to change/reschedule our trips and didn't really have too much of an issue, which would lead me to assume that you could purchase the tickets in person.

In your case, it could swing either way: let's say you purchase up front and select your days - what if it is a bad weather day and the next day is much brighter? On the other hand, let's say you don't purchase in advance but the day you are there, it is bright and sunny after three days of bad weather -
and they are sold out because of all the folks who waited the three previous days and have beat you to today's tickets? I think you'll have to decide by flipping a coin or some other way. It also depends on how many days you might be able to allot to being in the Denali area - popular wisdom is that if it is bad weather in Denali and sout of Denali, Fairbanks has bright sunshine! As for us, I played safe by purchasing tickets for three days and got lucky on the first!

Taking the early bus to Eielson is definitely the thing to do - you should plan for 8:30 at the latest. And even that will likely just barely get you all the way to Kantishna and back, i.e., if you plan to do nothing extra on your own (in other words, you'll go in and come out on the same Kantishna bus). The ranger experience at Eielson was not too great for me - for the kids, yes, but not for adults. But it was a wide open area with not too much fear of bears :-)

Coupon books: we were a group of five. The coupon books cater to specific interests and also are typically for a buy-one-get-one-free. Of all the coupons we saw, the only ones I thought we would be using were the cruise coupons, which gave - IIRC - $50 off the second person's ticket. Other coupons include one free ticket for the aerial tour, one free ticket for each of multiple train rides (but we could not split up the party for the train ride, or purchase three books for the five tickets which would have all added up to more expense and more time taken as well). But if you are in Perth and will be there in about ten days, isn't it too late to purchase the coupon books? [I don't know if they are available for purchase locally - I never even tried looking for the ones famous online or for any others in stores]

I never asked around online for the coupon books - that might have worked for us as well. That's a good option that I didn't even consider! But remember, IIRC you need to have the entire coupon book on you and not the loose sheets of the specific coupons you want to use. So make sure that you get the physical complete book(s) that has the coupons that you need to use still attached to it.