Some clarifications.
The spirit doesn't track damage, and doesn't have hit points. Instead, if it takes over a certain amount of damage in one hit, it is destroyed, similiar to a minion. You only take damage IF it is destroyed in this manner.
The spirit does -not- block line of sight, nor line of effect, nor is it cover in any way. Conjurations are not cover by default, and the spirit conjuration does nothing to change that.
The spirit does not flank because it is not a creature, unless you have a power that allows it to.
The spirit does not provoke opportunity attacks because it is not a creature, and it never takes move actions anyways. An opportunity attack is not a power, and does not explicitly target conjurations or zones. Neither can class features that are not powers affect a spirit companion.
Any temporary adjustments to your attacks and defenses -do- apply to the spirit companion.
The spirit companion is not affected by anything that isn't damage, and can only be targeted by melee and ranged attacks. EXCEPTION: Effects that explicitly affect conjurations (like Dispel Magic or Spirit Call) or that explicitly affect spirit companions (like Wrath of Winter) will affect a spirit companion just fine and with full effect.
A spirit companion cannot be given free attacks by a warlord, nor can it be moved by a warlord's powers, because it cannot be affected by non-damage effects. If, however, the warlord gives you actions, you can use those to use your powers as normal, which may or may not involve Spirit powers.
Any move action you take will permit you to move the spirit companion. This includes walk, run, shift. The spirit companion does not take a move action to do this, it simply moves those spaces. This movement works -exactly- like moving a flaming sphere or any other conjuration and uses the -exact- same terminology. The movement must remain within 20 squares of you, or else the conjuration ends.
When you die, the spirit companion goes away.
When applying a question to a spirit companion, always remember Specific beats General:
First, use the rules for the effects of powers...
...unless the rules for Conjurations override them....
...unless the rules for Call Spirit Companion overrides that...
...unless the rules for the specific Spirit power you're using overrides that.
Work your way up the hierarchy.
For example:
Does the spirit companion trigger OAs?
First: Do the effects of powers themselves trigger OAs? No. That's silly. Fireballs can't take OAs. But wait...
Second: Do conjurations themselves trigger OAs? Nothing specific beats the general here, so...
Third: Do Spirit Companions trigger OAs? Nothing specific beats general here... so we apply the general rule.
Another example of SbG:
Do -Summonings- trigger OAs?
1) Do power effects trigger OAs? No. But...
2) Summonings are creatures. Creatures DO trigger OAs. Therefore, specific beats general, and therefore Summonings -can- trigger OAs.
See? It's not hard.