1. Emacs Lisp is great (for the problem domain), exists and works. Common Lisp could work but then given how close Emacs Lisp and CL are, there's no clear benefit, especially with Emacs getting native compilation. Scheme was proposed, some POC code written and failed because nobody was interested. It doesn't fit and fragmenting one of Emacs's greatest strengths (consistent ecosystem of working Emacs Lisp code) is a terrible idea.
2. The Emacs UI core is completely toolkit-agnostic. It doesn't use GTK on Windows. It doesn't use GTK on macOS. You can run _graphical_ Emacs without GTK or any toolkit whatsoever on Linux, and not lose any functionality. Therefore what you write makes no sense (ugly hacks to support a subset of GTK).
3. There are multiple ways to do this. Read up on setting the mark.
4. Threading has nothing to do with TRAMP freezing, bad configuration does. TRAMP documentation (does anyone that bring up these points ever read it?) has multiple sections on improving performance. Check "SSH control master" and "Improving performance of asynchronous remote processes".
5. SVG support is already there and more is on the pipeline.
What I've noticed during my 40 year computing career is that folks get lazier and lazier. Few read manuals / documentation anymore but most are quick to jump to superficial conclusions without any substantial time investment in trying to figure things out. Spoonfed knowledge is the order of the day, and the folks doing the spooning are usually not far from clueless themselves (the blind leading the blind). This leads to generational gaps and rapid propagation of erroneous ideas amidst the newbie masses, very similar to what you laid out.
Emacs mastery requires commitment and time investment in order to understand (at least) the basic models and the trade-offs involved. If that doesn't take place, one can start going down paths that are entirely irrelevant ("Emacs needs threads so that it doesn't freeze!", "Let's use JavaScript so we can do non-blocking IO!", "Let's rewrite Emacs in Rust!")