Sounds like you’ve got it right. As long as you tell opensuse to mount your existing home directory somewhere besides /home you’ll be fine. Even if you do mount it at /home it won’t delete or overwrite it, you’ll just get lots of conflicts with the “foreign” pre-existing configuration files. So yeah, mount it somewhere like /mnt/home and opensuse will leave it alone and make its own /home directory on its own partition, and then you can symlink to your existing mounted partition.

As for grub, it depends on how you do it. If you go with the defaults you’ll probably overwrite kubuntu’s grub with opensuse’s grub. If so, opensuse will probably detect your kubuntu installation and make an entry for kubuntu. Especially if you’re not planning on keeping kubuntu around long-term this’d be the way to go. It’ll work if you want to keep kubuntu as well, but if you don’t want to keep kubuntu around definitely go with this.

Alternately, you could tell opensuse to not install grub when you install it. This would leave kubuntu’s grub installed and in charge, and then you’d go back into kubuntu and regenerate grub’s configuration with the update-grub command and kubuntu’s grub will detect your new opensuse install and add a menu entry for opensuse. This will keep your boot experience the same except for your new opensuse option, but you’ll have to keep kubuntu around since it’s still in charge of your boot menu.

Regardless of which option you choose, if you keep both operating systems installed you’ll likely have to go into the os in charge of grub and manually update grub every time you install a new kernel in the os that isn’t in charge of grub. It’s not hard, but it is another thing to remember.