  We report the first independent, second-epoch (re-)detection of a
directly-imaged protoplanet candidate. Using $L^\prime$ high-contrast imaging
of HD 100546 taken with the Near-Infrared Coronagraph and Imager (NICI) on
Gemini South, we recover `HD 100546 b' with a position and brightness
consistent with the original VLT/NaCo detection from Quanz et al, although data
obtained after 2013 will be required to decisively demonstrate common proper
motion. HD 100546 b may be spatially resolved, up to $\approx$ 12-13 AU in
diameter, and is embedded in a finger of thermal IR bright, polarized emission
extending inwards to at least 0.3". Standard hot-start models imply a mass of
$\approx$ 15 $M_{J}$. But if HD 100546 b is newly formed or made visible by a
circumplanetary disk, both of which are plausible, its mass is significantly
lower (e.g. 1--7 $M_{J}$). Additionally, we discover a thermal IR-bright disk
feature, possibly a spiral density wave, at roughly the same angular separation
as HD 100546 b but 90 degrees away. Our interpretation of this feature as a
spiral arm is not decisive, but modeling analyses using spiral density wave
theory implies a wave launching point exterior to $\approx$ 0.45" embedded
within the visible disk structure: plausibly evidence for a second, hitherto
unseen wide-separation planet. With one confirmed protoplanet candidate and
evidence for 1--2 others, HD 100546 is an important evolutionary precursor to
intermediate-mass stars with multiple super-jovian planets at moderate/wide
separations like HR 8799.

Earth and Planetary Astrophysics;Instrumentation and Methods for Astrophysics;Solar and Stellar Astrophysics