And poor Warnie! He was a scholar in his own right, he did try to beat the bottle, but it eventually beat him. There wasn't the support and treatment available in those days that could have saved him.
I think it's actually Lewis's adherence on most occasions to the idea of "mere Christianity" that makes it possible for so many folks to claim him! (and that's not necessarily a bad thing). He says in the foreword to Mere Christianity that the concept is like an entryway or hall to the various Christian denominations, and that you can't stay in the hall. But of course that's where the book is -- basic tenets that almost all Christians can agree with. In the words of a salesman, "Get 'em in the door!"
You consider his generation (that of my grandfather, WWI veteran) and how he was raised (an Northern Irish Protestant) and Lewis was remarkably free of religious bigotry, specifically anti-Roman Catholic bigotry. He was good friends with J.R.R. Tolkien after all, among other Catholics, and like good Anglicans, even (or especially) theological conservative ones, had a big-tent idea of “mere Christianity” as you said.
He, like most Anglican and Protestant types today, even evangelicals, freely admit its very possible to have a personal relationship with Jesus, and yet remain a Roman Catholic. While this may not sound like something big, just a generation ago this could not be said, evangelical types admitting RC’s can be born-again....and at that time neither would many RC’s reciprocate the idea of non-Catholics going to Heaven.... Times have definitely changed.
I hope this is a true ecumenicism, and not just a product of post-modern relativism...”if it’s good for you...”