Compared to C, rust compiles much slower. This might not matter on performant systems, but when resources are constrained you definitely notice it.
And if the whole world is rewritten in Rust, this will have a non-significant impact on the total build time of a bunch of projects.
unfortunately, the sdk doesn't work with hi applications yet. the way the screen works has changed significantly. There are community projects that make it work though: https://github.com/ddvk/remarkable2-framebuffer
The paper just describes a new method for selecting the main chain. Bitcoin uses the longest chain (with the most work) whereas this paper wants to use the block that is the child of the heaviest sub tree.
The ledger would still be a chain of blocks.
For real tree like blockchains there are a few examples such as IOTA, raiblocks and byteball.
Satoshi Nakamoto also intended for the block sizes to go up after an amount of time, yet the bitcoin core team isn't willing to upgrade and tries to promote their own agenda (blockstream).