>>4447027
I would bet those borders are caused by the glass reflecting internally, yes. Just get an adjustable easel or make one for the sizes you want to use.
Regarding wattage, for reference my D2 uses a 75W bulb and does up to 4x5. That'd be 15W converted down to 6x7 by area alone. Are your condenser lenses fucked somehow?
>>4447036
>By the way, /fgt/, behold my darkroom
behold