How can I get iOS to stop changing asterisks (*) to capital "I"s?
You can set iOS to not autocorrect something by adding it as an autocorrect shortcut. This also lets you see if autocorrect is to blame without disabling it.

Unfortunately, iOS doesn't let you set text replacements of less than 2 characters. OS X is supposed to not either, but you still can, and iCloud will sync the settings over.


Go to System Preferences → Keyboard → Text and click the + in the lower left.
For both the ‘Replace’ and ‘With’, type an asterisk.
Wait for iCloud to sync the settings over to your iOS device. Your new text replacement should show in Settings → General → Keyboard → Text Replacement.


If the asterisk still changes, there's likely something else going on here, and I would suggest disabling autocorrect entirely just for testing purposes to establish if it is the root cause of the problem.