I'm impressed that your oncologist checked out a possible benefit of Tamoxifen for MS.
You're absolutely right, the "estriol" hypothesis is just that, a hypothesis that it might be relevant to MS. There was a small Phase I Clinical Trial of estriol in women with RRMS and SPMS. The results were favorable for women with RRMS and a Phase II trial is planned.
Here's a link to the original study: Treatment of MS with the Pregnancy Hormone Estriol
It was later reported (Actrims 2003 Conference) that estriol also appeared to slow the development of those infamous "black holes" in women with RRMS.
highlights some detailed info about estriol and immune modulation.
Personally I think the potential of estriol/estradiol to help manage MS is via neuroprotection. That's also a hypothesis as the info is pre-clinical, but IMHO the pre-clinical evidence is good, i.e., some of the neurprotective properties of estrogens seem to mirror potentially relevant facets of the MS disease process. For example, there's been some recent discussion here about mitochondrial dysfunction and MS.
This abstract, Novel Mechanisms of Estrogen Induced Neuroprotection
Among the numerous aspects of brain function regulated by estrogens are their effects on mood, cognitive function, and neuronal viability. Here, we review the supporting evidence for estrogens as neuroprotective agents and summarize the various mechanisms that may be involved in this effect, focusing particularly on the mitochondria as an important target. Mitochondria Play a Central Role in Estrogen Induced Neuroprotection
Indeed, the parental estrogens and novel analogs stabilize mitochondria under Ca(2+) loading otherwise sufficient to collapse membrane potential ... suggesting that these compounds prevent cell death in large measure by maintaining functionally intact mitochondria.
Unfortunately I haven't read the article and don't know if one of the "novel analogs" they considered was Tamoxifen.
I have read only a tiny bit about selective estrogen receptor modulators (SERMS), including Tamoxifen, that suggests it does indeed have neuroprotective properties, but these properties are not necessarily identical to estradiol. For example, this research
the mechanisms of neuroprotection by SERMs and estradiol are not identical, because SERMs do not significantly affect reactive gliosis while neuroprotection by estradiol is associated with a strong down-regulation of reactive astroglia.
At any rate, please do keep us posted on how you do with it. If possible in your circumstances, you might also want to consider having your progesterone level checked. There's very little research, but there's some, which suggests it's the ratio of estrogen to progesterone (rather than absolute levels) that might contribute to MS symptoms and/or exacerbations.
At any rate it seems like the Tamoxifen might well prevent a recurrence of the breast cancer and help manage the MS too since it also seems to have some neuroprotective properties. I certainly wish you all the best in tackling both.