IPhone – find the magnifying glass UI for iOS 10

No - if you can't effect a repair using a clear film or some sort of adhesive with the phone intact, you'll have to totally disassemble the 3G/3GS display unit to get better access to the home button.

The teardown guides from iFixit are accurate - as is your reading of the difficulty of successfully doing the repair and having things work when put back together.

Do check with Apple on the replacement price of that part before you dive in. Last I checked, it was $99 for parts and labor for an entire new display assembly.

Unfortunately, This is not possible due to current App Store limitations unless someone else with a jail broken iPhone that you trust has a copy of it that you can grab. One problem with dealing with the Apple App Store is that older versions are forever lost due to Apple's App Store polices. We should be giving concerned feedback to Apple to address this issue too, not just the app developers. So it looks like your best option if you need to have the Mint app, is to read up on the latest jailbreak, and insure your jailbreak path, then update your iPhone to the latest iOS, and then re-jailbreak it. The good news is that there is a untethered jailbreak for both iOS 5 and iOS 5.0.1 for the iPhone 4 as referenced in my comment above.