The ambiguity is because of how the place was described in most of the records. The railway head was officially called Tadepalli, with the MG station also having that name. However, because of it's proximity to the South Krishna Canal, and the railway being used to feed the traffic on the canal, it was also referred to as 'Krishna Canal' in publications that weren't very detailed. Until the opening of the BG line, it was common to see both names being used in documents, though the official name of the station and the MG terminus was Tadepalli, and the revenue and survey records too only used the name Tadepa.lli
In fact, the bridge had a mixed gauge line right from the beginning...
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(traffic commenced on 17th March, 1893), with the BG line terminating at Sitanagaram (between KCC and the bridge) which had engine sheds for the East Coast railway (Sitanagaram station and the sheds were shutdown between 1903-1906). But note that until the present day KCC station was constructed, the BG terminated right after the bridge.
On 15th July, 1897, the BG section from the bridge till the present day KCC station, was opened. Note that, at that time, the term 'Junction' was used for stations where the lines from different railway companies also met (That's why Waltair and Howrah were initially called junctions). The line was opened till KCC so that transshipment from MG to BG freight could be done at KCC instead of BZA and thus, reduce traffic at BZA. This is the present day station, while the one marked as 'Kistna Canal' in the map you shared, is actually the Tadepalli station, which was a separate station at a different location from the KCC station.
You will see both KCC and Tadepalli stations being mentioned separately from about 1899. And KCC station never handled MG passenger traffic, even though the line passed through it. Tadepalli was the MG station and KCC was the BG station, with a minor transshipment yard at KCC for freight.
You can see this in the attached SR timetable from 1961, which does not show KCC station on the GNT-MTM MG timetable.

The MG station, loco sheds and freight yard were at Tadepalli.
The MG line branched out towards GNT just before the beginning of the platforms at KCC and thus there were no MG platforms at KCC. But a loop line was available that passed by the outskirts of the station, close enough that goods could be shifted from the MG freights to the BG freights.

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passenger trains, and freight trains bound for MTM or BVRT (thereby not needing any transshipment to BG) would just skip KCC station, just passing through its outer.
Anyway, going back to the point that started this, BZA station is indeed older than KCC station, but just by 8 years. And in general, the BG line reached BZA a few months before the MG reached the river from the south end.