How can you sing high notes without using alternative voice?
You have to merge your real voice with your falsetto such that your falsetto is indistinguishable from your real voice. It helps do scales that alternate between your real voice and falsetto. The key is to make a gradual change from your real voice to falsetto and vice versa and that involves lightening your sound when you approach your “breaking note” —- the note where you swtich from real voice to falsetto and rounding the sound when singing from the falsetto down to the real voice.