How do I enable 'Use Column Layout' in Apple Mail 9.3 on El Capitan and find a reliable alternative email app?

Running El Capitan MacOS 10.11.6 on an older iMac. Using Apple's "Mail" application, v. 9.3 (which is what ships with El Capitan). I've read online that v. 9.3 is the only version compatible with El Capitan. I've also read online that many users in recent years are having problems getting it to work reliably.

Three questions:

-- What happened to the "Use Column Layout" in the View menu in 9.3? I have various Macs with earlier and later OSes and hence earlier and later versions of Mail, and they ALL have "Use Column Layout" as an option. Was this option uniquely disabled in version 9.3, but not in any previous or subsequent versions? If not, then where/how do I access it/turn it on?

-- Mail 9.3 acts wonky, and occasionally will quit unexpectedly at random moments. I've followed several discussion threads here and other advice threads/articles/videos about how to fix this, and although these steps (such as trashing the plist file) improved the issue somewhat, the crashing still happens occasionally. I suspect the application file might be corrupted in some way and would benefit from being reinstalled, but I can't find any way to do that. Where/how specifically can I download and (re-)install just Mail 9.3, but not the entire operating system? (I have the original 10.11.6 install package, but it's "all or nothing" -- users not allowed to use it to install individual components, as far as I can tell.) I can't find the Apple Mail program in the App store, nor anywhere trustworthy/reliable to download from the Web.

-- Is it possible to just ignore Apple's "Mail" program and instead download/use a third-party mail application that works better? I would need one whose design/layout is very similar to Mail's layout (which is based on the old "Eudora" program design from decades ago). Do you have any recommendations for a good substitute email program that looks like Apple's "Mail" but works better, that will run in 10.11.6?

(And no, this is not about updating to a newer OS -- want to find solutions for/within El Capitan, not upgrade the entire OS.)

Any advice greatly appreciated. Thanks!

[Re-Titled by Moderator]