Hey I can help with a few suggestions (which may or may not answer to your questions)
From what I have read you seem to only choose one winner, and that will surely cause a certain disagreement among the other contestants. I often see artists who hold contests on Instagram, decide on multiples winners (2 or 3) and some honorable mentions. However if you insist on only one winner, you can kinda sooth the others with reposts of their submissions and tag them as a way to say 'Hey, I looked at your work, though you didn't win but it was nice'
It can happen,although you can always drag them back, eg. with interesting rules/requirements, or cool prizes, or if you notice a certain contestant that never won but kept joining, it doesn't hurt to give him/her a word of encouragement or thank to have joined. Plus there will be more and more people joining so I don't think you should worry too much, it's their choice
. You lose some but you also gain some.
To be honest I'm never a fan of voting. To avoid the bias I think you gotta make a personal selection of really good ones that deserve the attentions and then have people vote for those particular chosen ones.
And btw I think it's also better if you have one or two extra judges in case you are worry about your choice being too personal? (like you would always prefer something that suit your taste) A few different opinions would seem fairer to the readers.