What is this “more mature-engineer friendly path”?

You can only apply through one of the engineering institutions, not directly to the Engineering Council.  So that means following the process of whichever institution you've joined.

There's nothing to stop you transferring your IEng to another institution, and then re-applying for CEng through their process.  But don't assume it will be easier - just different.  Other institutions may be even less friendly to someone who doesn't have exactly the right degrees.

What is this “more mature-engineer friendly path”?

You can only apply through one of the engineering institutions, not directly to the Engineering Council.  So that means following the process of whichever institution you've joined.

There's nothing to stop you transferring your IEng to another institution, and then re-applying for CEng through their process.  But don't assume it will be easier - just different.  Other institutions may be even less friendly to someone who doesn't have exactly the right degrees.