Second, if a natlang were to be selected, what particular features
should that natlang possess to be a good candidate for an IAL? I would like to
hear what you think the important phonetic, orthographical, lexical and
syntactical features would be.
Nothing from the above except lexical. Even Ubykh is/was learnable.
As for lexical it should be immediately ready to use.
Humanity won't wait for academies to create new words for newly appearing concepts. The lexicon must be of the same size as of the native language of the student.
This reduces candidates to major language in use at the moment of such choice if we are talking about globalization.
If you need a lingua franca for tribes around Amazon river the language must similarly be able to express common concepts from all local languages in question. Unique concepts limited to particular languages should be borrowed with ease.