Isn't this the old concept of Ironman Style? I've finished Fallout2 like that a couple of times myself, albeit not on the Hard difficulty level.
I think it's a very very tall order that you're supposed to keep your companions alive. I don't think it's realistic. When you run into a big Enclave Patrol of 8 patrollers and you are busy trying to survive the fight yourself, and two or three of the patrollers choose to target the same NPC, there's nothing you can do. In an Ironman playthrough, sometimes you simply have to sacrifice a follower to stay alive yourself. When your character is in danger, it is sometimes the NPCs who save you by drawing some of the enemy fire to themselves.
I agree that Marcus is very hard to keep alive. In my opinion though, it's not his tendency to get knocked down that is the main problem. I think the main problem is how unmanageable he is. Unable to be told to stay close to you or to stay where he is, he often strays away from the main group, and then you cannot Superstim him if he gets into trouble. This is especially troublesome on the Oil Rig.
I agree with many here that the trip from the Den to Vault City is one of the most dangerous parts of the early game, as you have a young character with few hit points and no decent armor. Then the trip from New Reno to NCR is quite dangerous as Remants of the Master's Army can mess you up badly if they happen to get criticals, but at that point you already have a car and are less likely to get intercepted. The third massively dangerous traveling you have to do is obviously to Navarro. A big Enclave Patrol is one of the worst things one can run into. Deathclaws and Fire Geckos are dangerous too, but only to the NPC's, as you can simply run away from them, which doesnt work that well when the opponents have long range weapons such as Gauss Pistols (Enclave Patrols), Laser Rifles, and Rocket Launchers (Super Mutants).
I usually play Ironman with a very high Endurance and I pick two levels of Lifegiver as soon as possible. Things get a little easier once one reaches New Reno. Getting the damage threshold bonuses from the boxing ring and some Combat Armor from the army base will make things much easier. However, up until the point one has 500 hitpoints or more, one will ALWAYS have to make sure to kill the enemies with automatic weapons FIRST. Even if you have good armor and the unbalancingly good toughness bonus from the boxing ring, a single armor-piercing critical from almost any burst weapon will end your whole attempt. I had that happen once, I think I was several real-life days into my attempt and was wiping out one of the crime families. Then one of them happened to get a lucky critical with a 10mm SMG ten hexes away or so. End of attempt.