I think you will not be able to search for an iframe within an iframe without changing the document's "context" using something like

iframe.contentDocument

method. You can read more about that here

In your case, you need to grab all of the iframes recursively as you reset the contentDocument as needed. Something similar to this may work for your purposes

function getAllIframes() {
            let allIframes = [];
            let topLevelIframes  = document.getElementsByTagName('iframe');
            for (let i = 0; i < topLevelIframes.length;i++) {
                    console.log(topLevelIframes[i])
                    traverseDeeperIntoIframes(topLevelIframes[i])
            }
            function traverseDeeperIntoIframes(ctx) {
                allIframes.push(ctx)
                // Fallback for contentWindow
                const iframeContext = (ctx.contentDocument) ? ctx.contentDocument : ctx.contentWindow.document
                const tmpFrames = iframeContext.getElementsByTagName('iframe')
                console.log(iframeContext, tmpFrames)

                for (let i = 0; i < tmpFrames.length;i++) {
                    traverseDeeperIntoIframes(tmpFrames[i])
                }
            }
            return allIframes;
} 

        const allIframes = getAllIframes()

        // Iterate over this allIframes <Array> resetting the contentWindow at every context.
        // For example 
        function queryOverAllContentWindows(allIframes, cb) {
            for (let i = 0; i < allIframes.length;i++) {
                const ctx = (allIframes[i].contentDocument) ? allIframes[i].contentDocument : allIframes[i].contentWindow.document

                // Pass the iframe context into any arbitrary callback
                cb(ctx)
            }
        }