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.
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