1. if you have extra cash, always have your companions on missions. this gets you more materials, which you'll always need.
2. sure, stuff like Flux (scavenging), forget what the other professions need. solutions, cortosis (archeology). sure it can be vendor bought but it's FAR cheaper to get them via missions.
3. using Thermo Flux as an example..rich mission will bring in about 25-30. a critical will bring in 40+ flux
4. i almost never buy materials on the gtn, simply because the prices are usually stupid. sure you can pay 90k for a stack of Aluminum or Bondite, or you can get it free by farming in about 30 minutes. send companions on missions while farming, hoard what they bring in for crafting or sell on the gtn.
#1 I agree completely and in all candor running the daily HM FP will more than cover running your companions on missions while you are running that HM FP.
#2 at the lower levels it can be a wash (buying from vendor or running the missions yourself. At higher levels it pays to take the time and run the missions.
#3 dead on.
If you are willing to spend a little time checking all the stacks of materials you can find some REALLY good deals - I have found stacks of 99 going for 15000 credits (that's 151.51 per unit). When I am searching the GTM I immediately go to the last page of items and go forward because it is usually the stacks of 99 that have the best pricing per unit, but are often on the last pages because the GTM sorts by total cost not by cost per unit (something I wish they would change).
Further, in MMOs time and money are interchangeable commodities. If you are willing to spend the time going to a planet and gathering then you save credits. If you are willing to pay someone else to do the gathering and/or missions for you by buying off the GTM you save time. Sending your own companions on missions you save a little of both - you save money by not paying a premium for someone else to do the work for you and you save time by having your companions do the "gathering" while you do other things (HM FP, daily quests, etc).