Well, I guess you are aware those are what I'd call self-imposed restrictions. Unless I'm horribly mistaken, beast companions are meant to fight alongside their master throughout their career. The hit points and attacks of the beast is what the beastmaster build gets in place of the benefits of the other ranger builds. The ranger even gets a ritual to bring back his pet each time it dies, even if that is halfway through each and every encounter.
So I hope you don't get disappointed if I say I can't use a solution with that kind of balancing factor (your #3).
I understand your position.
I think you are mistaken though. As designed, companions should stay out of combat because they are about as tough as tough minions:
1) Average same level foe hits the average companion AC on a 9 or 60% (level + 5 vs. level +14 AC). Higher level foes hit more often.
2) Average same level foe (except Brutes) hits the average companion NAD on an 8 or 65% (level + 4 vs. level +12 AC). Higher level foes hit more often. It's extremely easy to put a condition on a companion.
3) Companions only get 2 healing surges. So if damaged a lot, the Ranger has to use his healing surges to heal it. This gets expensive quickly. And the Ranger can only do this if the companion is adjacent. The Ranger could use Second Wind to help, but that is limited to twice per day.
4) Raising the companion costs 50 GP (500 GP Paragon, 5000 GP Epic) in rare herbs or residuum. If the Ranger does not have these or runs out, the companion is on the sidelines.
A Ranger gets 6 + Con Mod healing surges per day. Even with a Con of 14 (which not all Rangers will have), that's 10 healing surges to share between the Ranger and the easy to hit companion. That's an average of 4 (low Con Ranger) to 5 each, knowing that the companion will get hit more often than most other PCs.
4 to 5 healing surge average means that the Ranger is going to be wanting the group to take an Extended Rest a lot more often. Sure, they might do that, but there are PCs out there with 10 or more healing surges who will not want to do this.
The only up side for a companion is that it typically has more hit points than the Ranger. But even this does not mitigate how easy they are to hit.
And since companions are melee creatures, they are easy to get caught in blasts or bursts.
Now, the Ranger could say "I could care less about my companion. I will carry a boatload of Rare Herbs and just let the companion die without using my own healing surges and then raise it". Course, the companion gets the -1 raise dead penalty until 6 or more encounters have gone by (3 milestones). And this is not the best of roleplaying either unless the Ranger is Evil.
It's one thing to add another PC to a group. PCs have a lot of combat options and can pull their own weight.
It's another to add an in melee companion that needs to be healed more often than any other PC on average, has very few healing surges, does minimal damage, and never adds any conditions to foes.
So yeah, using the companion sparingly is a self imposed restriction. Not doing so is extremely poor tactics that will cost a lot of resources. Pros and Cons, hence, balance.