Hello robinsonone. Good to meet you, and so sorry it has to be under these conditions, but hey, let's see if I can help you out here.
1. Absolutely. The concentrated cannabis oil will help counteract many, if not all of the side effects from radiation and chemo. It may take more, because chemo in particular comes at such a high physical cost that the cannabinoids have to work harder.
2. Dry mouth might prove to be a benefit with tacking, since one of the chief challenges is getting the mouth dry enough to grab the oil.
Suppositories would offer you not only dosing without euphoria but also a way to increase the dose more dramatically than just about any other method. As a bonus, breast cancer responds well to suppositories as a delivery system. Here's a link to cajuncelt's cancer protocol thread. There's a wealth of information in those pages, including important information about making the oil more bioavailable. I highly recommend you take the time to check out that thread.
Suppositories
3. I'd think any concerns about vertigo would be related to euphoria, and that might be a legitimate concern, but you're looking for ways to avoid problematic euphoria. The vertigo shouldn't be made any worse with oil, and may improve, given the many ways the cannabinoids interact with the human systems. Many of the cannabinoids fight infection and counteract nausea and the endocannabinoid system regulates elimination, so that should improve with the oil usage as well.
I hope I answered these to your satisfaction. If not, please, keep asking. You should pepper us with questions until you're comfortable with proceeding. It's a dedicated band of cannabis warriors that prowl these halls. You're in good hands. There's much to be hopeful for. Please extend the warmest wishes for a speedy recovery to your mother.