If you have replaced battery and dock, Fast battery drain, red screen, and sudden death are all very good indications of a short.
More often than not the short is the tristar IC (1610A2), which becomes damaged from using cheap wall chargers, car chargers, ipod docks, ect.
The issue is board level, and its impossible to know if the issue is this ic or something else on the baord. Your best bet is to send it to someone for board level diagnosis or repair. There is a link on my profile to a website that will help you locate some of the most trusted board level techs there is. There may even be one close to you. If not, all of them accept mail in repair.
Sudden death on the iPhone 6 series is common, and it can almost always be fixed, but only by experienced board repair technicians. I fix many of these each week