Hi James
Firstly, ISNUMERIC() has limitations - it returns true (1) if the value can be converted into a number. SSC has a recent article covering its limitations.
Secondly, the filters in the expression may be evaluated by SQL Server in any order, not necessarily the logical order which you've carefully calculated should work.
Thirdly, a few rows of column QR_FreeText should help folks determine the best way to solve this problem. A few rows which should convert, and a few rows which should not.
For fast, accurate and documented assistance in answering your questions, please read this article.
Understanding and using APPLY, (I) and (II) Paul White
Hidden RBAR: Triangular Joins / The "Numbers" or "Tally" Table: What it is and how it replaces a loop Jeff Moden