In my opinion you need not to separate /home and / partitions if you don't really need. Rest is all your wish. You can read Ish's answer.

The warning states that:

No partition table changes and no creation of filesystems have been planned. If you plan on using already created file systems, be aware that existing files may prevent the successful installation of the base system.

Actually, Linux(let's say Ubuntu here) works on extended file system which is quite obvious that it is a standard. You previously had Ubuntu installed in that partition(lets say sdax). Since you already have that partition with ext4 filesystem and you are just continuing without any changes or creation of File System. It expects that you might have partition in another file system and expects you to change or create a new file system.

Like I already said that Ubuntu works on ext file system and it might expect you to change filesystem or at least format your partition(you haven't checked on Format this partition box). That's why be aware that existing files may prevent the successful installation of the base system.

So, just choose Format this partition and install Ubuntu as usual.

The warning simply means that you won't be erasing all files on the existing filesystem, but instead overwriting a selection of them. If you have a truly haywired or strangely-customized system, this may cause problems since the installer is not omniscient and will not try to understand your changes.

In ordinary usage, you are unlikely to encounter problems. I've used this install method many times without problem...but I tend to overwrite an uncustomized, stock Ubuntu with another stock Ubuntu.

Since you have backups, it's safe to select the option to Format the partition, which will delete everything and start you fresh.

The bug you are worried about was fixed eight years ago. Since you are not installing 10.04, it is very unlikely to affect you. If you are worried about some similar kind of behavior, that's reasonable. Address that worry by having backups, recording your Win 10 Product Key on paper, and knowing where to get Win 10 reinstall media from the MS website.

It is unwise for new users (like you) to follow random advice you find on unofficial websites and blogs (like howtogeek). They are full of opinion, rumor, shockingly bad advice, and obsolete ideas. There is nothing wrong with keeping / and /home on the same partition. That was a common practice 10-15 years ago for a lot of good reasons, few of which apply today - disks are bigger and OS are more reliable. There is also nothing wrong with keeping them on separate partitions. As a new user, feel free to try both (it's allowed)...and regardless of which you choose as your personal preference, keep good backups.

The bug you mention was fixed about eight years ago (see toward the bottom of your link).

A reinstall requires erasing things on your hard drive, yes, but only your old Ubuntu installation. It doesn't require erasing Windows.

Before doing anything, back up anything you want to save from Ubuntu, maybe in Dropbox or something like that or a USB drive. You might also consider doing the same with important files in Windows.

Also, when working below, do NOT do anything to ntfs partitions.

Getting a separate /home partition

A separate /home partition is usually not recommended because it wastes space (when one partition is full, it can't use the space in the other), so you should do it only if you have a good reason to.

If you want a separate /home partition, delete the ext4 partition by selecting it with your mouse and pressing the - button in the lower left. Then, click on the new free space, click the + button, and create a / partition that takes up only part of the free space. Then, click the remaining free space, press the + button, and create a /home partition with the remaining space. Format both of them if it gives the option to.

Formatting the / partition

If you skipped making a /home partition, do the same thing you did before, except check next to Format the partition in the picture below and set the mount point to / again:

This will completely erase /dev/sda5 that contains your old Ubuntu installation. So backup any files you need from your old Ubuntu installation onto your Windows partition or something like Dropbox or a USB drive, because they'll be erased and you probably won't be able to get them back.

However, it will erase only that partition (as it says, Format the partition), leaving alone other partitions like /dev/sda3 that contains Windows. That will make the base system installation work nicely while not messing with Windows.