  The Dragonfish Nebula has been recently claimed to be powered by a
superluminous but elusive OB association. Instead, systematic searches in
near-infrared photometric surveys have found many other cluster candidates on
this sky region. Among these, the first confirmed young massive cluster was
Mercer 30, where Wolf-Rayet stars were found. We perform a new characterization
of Mercer 30 with unprecedented accuracy, combining NICMOS/HST and VVV
photometric data with multi-epoch ISAAC/VLT H- and K-band spectra. Stellar
parameters for most of spectroscopically observed cluster members are found
through precise non-LTE atmosphere modeling with the CMFGEN code. Our
spectrophotometric study for this cluster yields a new, revised distance of d =
(12.4 +- 1.7) kpc and a total of Q = 6.70 x 10^50 Lyman ionizing photons. A
cluster age of (4.0 +- 0.8) Myr is found through isochrone fitting, and a total
mass of (1.6 +- 0.6) x 10^4 Msol is estimated thanks to our extensive knowledge
of the post-main-sequence population. As a consequence, membership of Mercer 30
to the Dragonfish star-forming complex is confirmed, allowing us to use this
cluster as a probe for the whole complex, which turns out to be extremely large
(400 pc across) and located at the outer edge of the Sagittarius-Carina spiral
arm (11 kpc from the Galactic Center). The Dragonfish complex hosts 19 young
clusters or cluster candidates (including Mercer 30 and a new candidate
presented in this work) and an estimated minimum of 9 field Wolf-Rayet stars.
The sum of all these contributions accounts for, at least, 73% of the
Dragonfish Nebula ionization and leaves little or no room for the alleged
superluminous OB association; alternative explanations are discussed.
