  Accreting protoplanets represent a window into planet formation processes. We
report H{\alpha} differential imaging results from the deepest and most
comprehensive accreting protoplanet survey to date, acquired with the Magellan
Adaptive Optics (MagAO) system's VisAO camera. The fourteen transitional disks
targeted are ideal candidates for protoplanet discovery due to their wide,
heavily depleted central cavities, wealth of non-axisymmetric circumstellar
disk features evocative of ongoing planet formation, and ongoing stellar
accretion. To address the twin challenges of morphological complexity in the
target systems and PSF instability, we develop novel approaches for frame
selection and optimization of the Karhounen-Loeve Image Processing algorithm
pyKLIP. We detect one new candidate protoplanet, CS Cha "c", at a separation of
75mas and a Delta mag of 5.1 and robustly recover the HD142527 B and HD100453 B
low mass stellar companions across multiple epochs. Though we cannot rule out a
substantial scattered light contribution to its emission, we also recover LkCa
15 b. Its presence inside of the cleared disk cavity and consistency with a
forward-modeled point source suggest that it remains a viable protoplanet
candidate. The protoplanet PDS 70 c was marginally recovered under our
conservative general methodology. However, through targeted optimization in
H-alpha} imagery, we tentatively recover PDS 70 c in three epochs and PDS 70 b
in one epoch. Of the many other previously-reported companions and companion
candidates around objects in the sample, we do not recover any additional
robust candidates. However, lack of recovery at moderate H-alpha contrast does
not rule out the presence of protoplanets at these locations, and we report
limiting H-alpha contrasts in such cases.
