Ishida's purity as a quincy according to bach is his (bach's) own judgment. He took powers of those he 'deemed' impure.
Ishida has been given the Sternritter designation 'A', a letter which has a meaning that is as of yet unknown.
Ishida being 'the last quincy alive' possibly means what bach said that time to yamamoto was more literal than we thought 'the quincy died that day, along with the shiningami'. The shiningami bit was metaphorical, but maybe they were truly massacred by the shiningami. The shiningami do not have the abilitty to completely destroy souls, so the quincy's dead souls most likely survived and went into hiding for this revenge.
It seems dubious but even the dangerous rogue first shiningami substitute Ginjou Kuugo and the others were left alone to wander after they died.
Bach is clearly stepping down, possibly due to his age or injuries from battle, who knows, and now the title of emperor is up for grabs. Haschwald has his own inner faction.
Ishida was represented a sakazuki cup of blood to give him powers. Perhaps Bach's cup itself?
Yes, a lot of speculation which isn't fact until proven, but in light of this this chapter was actually pretty intriguing rather than confusing.
It was definitely short though