Install grub-customizer. There's a convenient way to set the default entry in the General settings panel. Then make sure you update your grub configuration (I believe this is done automatically once you save the new settings, but I'm not sure). Otherwise, you can add these two lines to /etc/sysconfig/grub:
GRUB_DEFAULT=saved
GRUB_SAVEDEFAULT=true
then update grub, so it will always remember the last choice.
You can lock a kernel version with the versionlock plugin, which has to be installed (the package is python3-dnf-plugins-extras-versionlock):
sudo dnf versionlock add kernel-4.18.16-300.fc29
To remove the lock just run the same command with delete instead of add.
If you also need to block any future update to the kernel you can add exclude=kernel* to your /etc/dnf/dnf.conf.
To install a specific kernel you need to download all the related packages from koji. For example:
koji download-build --arch=x86_64 kernel-4.19.15-300.fc29
provided that you have the koji package installed, this command alone will download the kernel* packages for the x86_64 architecture in this case, debug packages included. You can then install the ones you need from the command line with dnf just as usual.
To file a bug report about the kernel I'll suggest to just refer to the usual guidelines.
