How can I securely install an older version of Chrome on a device that won't support newer versions?
Give this a try:

http://www.apkmirror.com/apk/google-inc/chrome/chrome-42-0-2311-111-release/chrome-42-0-2311-111-4-android-apk-download/

I use APK mirror all the time and have never had any issues.  You said 42 and below right? This is version 42