For those who need to gain weight

IMO gaining 15-20lbs in 3-4 months wouldn't be healthy. At most I'd expect you (female, beginner) to be able to gain about 1lb of lean mass per month so you'd be talking about gaining at least 10 lbs of fat with whatever muscle you add. Gaining strength and muscle is a longer term proposition than 3-4 months, I suggest you reassess your expectations and timeframe.


Source: https://www.precisionnutrition.com/rates-of-fat-loss-and-muscle-gain#musclegain

I'm also confused by your exclusion of "body builders" from giving advice because, like it or not, building one's body is the process you described. I would speculate that comment is born out of fear of "getting too bulky". I'd suggest this thread below, full of posters who have been successful improving their body composition.
https://community.myfitnesspal.com/en/discussion/977538/halp-heavy-lifting-made-me-supah-bulky/p1