If this were your plant, what would you be doing t...

Yes any deficiency is a concern and need to address immediately. U can go to growweedeasy.com they have a plant diagnosis with pics and solutions. After I solved problem then I would strip all big fan leaves not the top. Completely lollipop everything where u have them and may go a bit further up. Give 3 days send her into flower.

Yes those brown spots are not normal and means you have something going wrong. To me could be nutrient burn or ph burn as the burning are not visible from the lower to the upper leaves which would have been the signs of a mobile elements deficiency ( like nitrogen , phosphorus or potassium) , if it's not a nutrient excess or a ph fluctuation ( 6.5 is a bit high to me but I am a 5.8 ph grower so that must be what makes me think that ) , I personally think that your ph and Ec values are late flowering values but you should still use the mid flowering values ( your plants aren't about to finish flowering , to me they are barely starting ). If I were you I would have low my nutrient input , low my ph to 6.0 , I would have trim the fan leaves that are under the canopy a d ate not receiving enough light to transform it into energy for the plant , If I were you I would have start asking myself "why these white widow aren't making bugs after 4 weeks of flowering" , If I were you I would definitely have found a way to have a higher temps during daytime than during night time ( I still don't understand how is it possible for you ? Are you using a big house heater during night time ? That kind of temps differences is unnatural and to me it can stress the plant ). That said if it isn't a mobile nutrient def or excess it could be an immobile nutrients def or excess and as I am sure that you are smart enough to find it by yourself I am providing you a link to a def/excess chart in the comment section