A. Make sure your shoes are wide enough for your feet. Start going barefoot as much as you can.
B. Heal your gut. If you have bad digestion issues, it won't really matter how much you are eating, as there is little chance that you are getting what you want out of it.
1. If you need to eat more calories, then adding fat is a pretty easy way to do it. Upping carbs can do it too. Cook your veggies in liberal amounts of coconut oil or butter. It might take your body a little bit to get used to eating more, but it'll help.
2. Don't do dairy then. Drinking calories does make them go down easier. Do you have a blender? I'm sure you can come up with your own blends of fruit and coconut milk that taste reasonably okay.
3. Short answer: NO! You can build muscle. You don't state your age or gender, but I'm guessing female (men don't have the tendency to wear overly-narrow shoes and get bunions) and middle-aged, or at least approaching it, from your comment about how long ago college was. Building muscle does rely on hormones, one of them being testosterone. You probably don't have much at the moment, but the more you increase your muscle the more you will develop. Lack of muscle at your level is simply a lack of stimulating the muscles to grow. You need to work your muscles, and eat to feed them.
4. I don't know what your specific issues with your neck and shoulders are, but you need to find someone who can tell you, and who can help you find ways to fix it. Kinesiologists, chiropractors, physical therapists, Pilates instructors, yoga instructors, Alexander Method trainers, ...etc. There are a lot of people who have the training who should be able to help you fix yourself. If it is as bad as you say, it's not really something that can be really fixed over the internet. Something as small as holding your head and shoulders in a correct posture might be more than you can do right now. Find an expert to help you. If they aren't helping, fire them and find a new one. Yes, it might be expensive. How much is a functioning body worth?
5. Protein makes you feel full. It makes pretty much everyone feel full. Some people get really full from fat. Some people get really full from carbs. You need to figure out which of those you can eat more of, and eat more of it. You are trying to gain weight. Have people add food to your plate when you aren't looking. Eat before you get hungry. Don't be afraid of the primal "gray area" foods, like rice, if your stomach can handle it.