Comment 28 for bug 1854798

@vivkar, in response to your last question, you should have kernel 4.15.0-70-generic already available to boot from, since an update wouldn't have replaced the old kernel version. Make sure it's not available by going to /boot/ folder on your machine and looking for any files named 4.15.0-70-generic.

If it is not available, perhaps you can try manually downloading the kernel version and then following the instructions in https://superuser.com/questions/208502/how-to-add-a-new-kernel-to-grub2. I'm a complete linux newbie, so take my advice with a grain of salt. If the the 73 version of the kernel is working for you right now, I think you should stay on it until an update becomes available.

I'm not sure about the answer to your other questions