Personally I'd go with a backing. Get a working bow out of it first, then remove the backing and see if it holds, then make one without.
Your tillering process would work fine for yew as it's such a forgiving wood, but with these funky meanewoods you have to be so careful to spread those stresses early and evenly.
The long string is a bit of a controversial one as well - many bowyers who make superb heavy bows don't use long strings, especially not for a 100lb bow. Something that light can be floor / vice tillered to begin with, then braced as soon as possible.
If you watch Ian Sturgess' recent video you can see the process clearly. It's a case of ensuring perfect tapers and watching the mass, getting the bow virtually finished before it even sees the tillering tree. This minimises set, and drastically reduces the strain put on a bow early on, while many other bow makers drag it down on a long string to see where the problems are - by which point those problems have already damaged the bow.
However, all of that said, with unknown woods and unfamiliar draw weights a long string does keep things safe, at the risk of having a lower performing bow. I think for what you're doing (at this stage just trying to make the thing work!) a long string is wise, but perhaps only to brace height. Provided the bow is fairly even at brace height, you should be able to see almost all the problems with the tiller from the full brace shape.