I was having a look at John Brit High Fire book today and it has a little section in there about single fire if you have the book. It did say that high clay content is needed for leather hard glazing but most glazes could work on bone dry pots as they have done the shrinking.
They might look better than usual, you never know. At least you found it before it was a 20 hour hold. It could have even worked out at your cone 5 down 6 half way assuming the first time it did hold and you have only added 27min.
It is interesting, the clay sold as terracotta has a firing range up to 1220 but all stoneware goes up to 1260/1300.
I think you are probably right that clays sold as stoneware are cone10 clays even if their range goes from 1080-1280. That doesn't mean there is not one out there that would suit cone6 but I have never found much useful data about the clays out there.
I would try a terracotta rated to 1220 and see if you have any luck.
There is one here rated to 1120c but also says 'will vitrify at high stoneware temperatures' after having a look through valentines clay it looks like a few possible cone6 bodys could be filed under white earthenware/terracotta.
"KGM Body can be fired 1080-1120oC
White to Off White at higher temperatures.
Smooth. Cost effective white earthenware clay which will vitrify at high stoneware temperatures. Due to very fine grog will give a light speckle at the highest te
"Potclays 1141 Studio White Earthenware (LT25)
Smooth. Ivory white very plastic white clay for throwing.1100-1220oC"
Potclays actually give this clay a vitrification range of 1180-1250 but the firing range above. Who decides to put these things.
It does probably have quite high expansion with all the soda feldspar. Lithium will help the crazing. If you have no lithium carbonate you could swap out say 5 of the soda feldspar for a lithium feldspar (if you have that) and see if it fixes the crazing.
Is it leaking out through the bottom? Could be that the clay is underfired even at cone8. Usual rubbish from the company that "Scarva Earthstone Original White matures between 1180°C-1280°C"
I have seen clear glazes in oxidation tend towards blue in reduction firings on porcelain. Not sure what the cause of it is but reduction brings out the blues.
Here is a quote from digital fire in the blue section, any iron will seem to send the glazes blue in reduction. I think that is giving the blue hue throughout the glaze with cobalt sending the glaze brighter blue.
Oxides - Fe2O3
In reduction glazes Fe2O3 tends to fire bluish or turquoise to apple green with high soda (boric oxide may enhance). 0.5% iron with K2O may give delicate blue to blue green.
They also say "Iron glazes will move toward blue if alumina is low."
Bit more reading and I found this on another website.
"The cup (fig 4) with iron glazes was fired under reducing conditions. The glaze on the inside contains 0.5% iron(III) oxide and the glaze on the outside 10%; the reducing power of the CO was enough to convert the iron in the inner glaze into Fe(II), but the concentration of iron in the outer glaze was just too high."