Dear Itai and Neil,
This is such a problem in Kenya, I tried to sort it out with Don a
couple of years back but we came to the conclusion that only a devoted
serious study could sort out the identification problems associated
with this group.

The real charmosyna, which I believe is a distinct form, is a northern
lowland semi-desert species. In general appearance it is a dainty,
very pale small bird, not the bulky sluggish form of erythronotus. In
the field the only visible red on the entire bird is the rump, whilst
erythronotus show extensive red on the underparts and in Nairobi on
the extreme northern edge of the erythronotus range (kiwanukae), the
flanks are bright red, and the entire underparts of the males are
suffused red, and the bird is extremely dark all over apart from maybe
the pale grey crown. Heading westwards away from our isolated
population, we come to the race delamerei in southern Uganda, to
Eastern Rwanda, and I once had one near Busia all on its own. This
race is really pallid, hardly any red on flanks, and the underparts
grey with hardly any indication of a rose hue on the underparts. Going
south, this bird extends across from Rwanda to central and eastern
Tanzania. Isolated kiwanukae, is the most spectacular of the whole
group, restricted in range from Nairobi south as far as Olduvai, and
very very strangely given as Dodoma (where presumably it is very close
to the washed out delemerei). Geographically a small range. As well as
the bright red of rump and flanks, the underparts have an entirely
rosy hue becoming red on the lower breast. Overall the bird is
extremely dusky.
Tsavo is a real problem, some birds are built like kiwanukae, but are
much paler and have reduced black on the chin. Your birds from Sinya
look much like these birds, but it is difficult to see from the image.
These TZ birds look like charmosyna to me. Overall pallid colouration,
neatly rounded not angular cheek patches, silver border to lower edge,
hardly any black below the bill, hardly any red on the flanks and
whitish undertail coverts.
Personally Itai I think you should start preparing the record of
charmosyna for the Tanzanian list. If you compare your birds to the
ones I photographed at Sagala Lodge in Tsavo East, you can see that
Tsavo birds show sharply angular cheek patches, a rosier wash and look
bulkier (if you can judge this from an image like these). They are
more like a pallid pale undertail-coverted version of kiwanukae. It is
possible that these pale birds are forms of female kiwanukae in the
dry zones of Magadi and Tsavo, whereas they are darker in higher
rainfall areas. I suggest this because I cannot otherwise find an
explanation for the appearance of the Tsavo birds, and Nigel Hunter
and myself watched a mixed pair of kiwanukae tending the same nest on
the Magadi Road a couple of years ago, where the male was typical
colourful kiwanukae, and the female one of these pale birds like those
of Tsavo. However from your Sinya image I am not sure that you could
rule out the birds as extreme northern delamerei, the female of which
according to “Finches and Sparrows,”  Clement, Harris and Davis 2003,
has pale undertail coverts. Then they go and state that some authors
consider that kiwanukae is a race of charmosyna and not
erythronotus!!!!!
So you can see the Black-cheeked/faced Waxbills are a real headache,
is there one or in fact three species?
I think the truth is that people do not look at members of the group
critically when they meet with them, assuming that they are all
Black-faced until they are in the range of Black-cheeked, and with
more observers paying more attention maybe we could unravel the mess
that we are certainly in now. If kiwanukae is conspecific with
Black-faced, then that species is still only found along the extreme
south of Kenya, and in fact very few localities, being common only
around Nairobi. For us charmosyna has a much more extensive
distribution. On the other hand if kiwanukae is sunk into charmosyna
as well, then my record from Busia of the bird resembling delamerei
would be the only record of Black-faced Waxbill in Kenya. In Tanzania
you would suddenly have both species, and I would be having to write
up a new bird for Kenya! We should probably have a look at the birds
in eastern Mara, as it is possibly that these birds could also be
delamerei, but I have no images from there, and they are very rarely
encountered. Have you any Serengeti images?

Trust this helps even if it only highlights the problems,
Brian