First Problem

if you are using jquery its fix simple.

ons.ready(function() {

   $('body').height(window.screen.availHeight);

 });

or pure javascript

document.body.style.height= window.screen.availHeight;

Second Question

Monaca localkit debugger using cordova version 10 now, if you are use this tools and you want view correctly please update your apps.

Maybe you dont want update your apps, okay you can build from Monaca Cloud IDE Build iOS App > Buil for Debugging > Custom Build Debugger and continue development.

Third Question

I haven’t tested this, but the splash screen doesn’t work correctly.
My own application, Default@2x~universal~anyany.png uses a single file, but when opening the application, first the “monaca” logo appears for half a second, and then my screen png appears. This problem should be fixed.