The NROTC 4yr scholarship award does come as a letter and in a thin white envelope but it is fat - as i recall. It is a multipe paged letter accompanied by at least a form that must be returned by a certain date. The year my son received his - received 3rd week of november with deadline date of december 31st. My son did also receive group email from recuiter who had processed his paperwork, that came about 3-5 days before the envelope. His on line did show he had been awarded the scholarship which triggered a DODMERB account. Since he had already done the DODMERB thing for USNA it was already in play and did not have anything additional to do for that part.
As for the question about other NROTC scholarships - my understanding is once the candidate is a college, one should enroll in NROTC class (1 credit hour) and PT - and apply for remaining years scholarship. I am sure the unit at whatever college one would be attending would be delighted to discuss and guide you thru the procedure.