I have been in your mother's shoes.
The words
fear and
scared have come through in your post fairly loudly.
It certainly is scary to get a diagnosis like this, but I hate to see fear as the reason for making certain decisions. I'd much rather see knowledge and confidence in decision making.
It's important to truly understand things very well before making final decisions that impact you forever. My doctors told me not to rush into surgery because there were things I needed to learn about and think about long and hard before I made my final decisions. It was solid advice.
See, the type of surgery you choose doesn't determine whether you live or die from this disease. If it did, most of us would be walking around without breasts.
Lumpectomy with radiation has been proven for many years to be equally as effective as mastectomy in preventing cancer recurrence in the affected breast. When discusssing my own options with my oncologist, looking at the film of my breast she placed her thumb over the tumor and said, "This is your tumor, the rest is healthy breast tissue. Why would you want to get rid of all that healthy tissue?". My reply? "Well I don't, but I want to be here to raise my children".
It was then that I learned that the type of surgery I chose would never guarantee me that

because the problem with invasive cancer is that some cells can get away from the tumor site before it's found, and settle and grow elsewhere later on, like the liver or brain, and essentially that's what you can die from. But again, that happens before you find the tumor, and is what chemo is designed to help, i.e. killing off those cells that got away. (Note this does not apply with non-invasive cancers that don't travel anywhere.)
In the case of mastectomy, and double mastectomy, one is a big surgery and two is even bigger. Then you are left with no breast tissue* and are usually looking at another major surgery for reconstruction. (I say "no" but in fact it's impossible, even with mastectomy, to remove 100% of breast tissue, which in actuality goes down as far as under your arm, and breast tumors have been known to crop up in scar tissue so even having mastectomy can be a false sense of security that breast cancer will never return and it's devastating to the person when it does. I was given the figure that there was a 2.5% chance of recurrence yearly even with mastectomy, and a 5% chance with lumpectomy - so with both types of surgery there is always a chance of recurrence) Sometimes when we're in this situation all we can think about is getting rid of the cancer, but we're not thinking about down the road with body image issues, how our clothes fit, how our spouses view us, pain issues, sensuality/sexuality issues, etc. (Not that these mean everything but they are things to be considered, for sure, because they do impact our lives later on.)
Some people do know all the facts and choose to have a double mastectomy anyway in order to drastically reduce the chance of a recurrence to the breasts forever. Which is great as long as you understand why you're doing it, that nothing is guaranteed 100%, that you can still ultimately succumb to BC, and that you want to choose to go forward with it anyway. (Survival, however, has come a very long way and most people diagnosed today will survive it thanks to early diagnosis and better understanding and treatment than ever. Even some advanced cases today can be treated more like chronic illness than death sentences. Coming to terms yourself with the diagnosis is still the hump that is difficult to get over, and certainly takes a lot longer than a month or two.)
However, in the situation you've described, it could be overkill, especially if the tumor is not invasive. Lumpectomy with radiation for non-invasive tumors has almost a 100% cure rate.
If the tumor is invasive and well defined, she could still have a lumpectomy (if that was presented as an option to her). Once the tumor is removed a pathologist examines the entire thing under a microscope to determine if all the margins are "clean", ie there are no cancer cells along the edges that could grow later. All edges need to have healthy tissue so healthy tissue can grow back. I chose a surgeon who was very precise (and slow!) who got all clean margins the first time. Done deal, cancer gone, home that night, still have breasts. Eight years later my breasts are still cancer free.
With all that said, there will be people that understand all of this completely and still choose to go forward with single or double mastectomy
or who feel better (less anxious) having the whole breast removed, and that is their prerogative. But at least know the facts before you make the choice.
That is the surgery part. I would strongly suggest that if your Mom needs more time to do more research or talk to her doctors again, ask questions, etc, she should take it, even if it means rescheduling.
As for the other part, ie invasive tumors. I still don't know if your mother's is invasive or not. But regardless, basically these days they can determine the likelihood of recurrence and make recommendations about chemo or not before the surgery. They'll still check the sentinel node to see if cancer has spread there and if not, that's it. No chemo. If nodes are positive, usually chemo is recommended. Because cancer traveling up from the tumor site to the lymph node means cancer was in the lymph/blood system and therefore, at least in theory, could have traveled elsewhere as well. Generally things will not show up on CT and other scans right away, it could take years. So to prevent the likelihood of further growth, they hope chemo will kill off the cells that got away.
I am the OP of the breast cancer thread you're looking for. You can find it under my username. Please PM me if I can be of further assistance, and we hope to see you on the other thread. We've had many "daughters" there over the years who just want to be supportive to their moms.