Well, I need to brush up on Heal Skill, so this is gonna take a while, since I'm going to end up over-analyzing this (I'm using d20pfsrd, in case there's any confusion).
By RAW, you can do the following things with the Heal Skill:
- Identify Drugs/Pharmaceuticals
- Provide First Aid
- Provide Long-Term Care
- Treat Wounds From Caltrops & Such
- Treat Deadly Wounds
- Treat Poison
- Treat Disease
The Heal skill is used to identify and understand pharmaceuticals. No further information was provided in the source material.
This would target the drug/Pharmaceutical, not a character, so it's moot for this purpose.
You usually use first aid to save a dying character. If a character has negative hit points and is losing hit points (at the rate of 1 per round, 1 per hour, or 1 per day), you can make him stable. A stable character regains no hit points but stops losing them. First aid also stops a character from losing hit points due to effects that cause bleed.
You can't use the skill if you're at negative hit points (because, dying), but you could use it to stop the "Bleed" effect on yourself. (See: "Rambo, John")
Providing long-term care means treating a wounded person for a day or more. If your Heal check is successful, the patient recovers hit points or ability score points (lost to ability damage) at twice the normal rate: 2 hit points per level for a full 8 hours of rest in a day, or 4 hit points per level for each full day of complete rest; 2 ability score points for a full 8 hours of rest in a day, or 4 ability score points for each full day of complete rest.
You can tend as many as six patients at a time. You need a few items and supplies (bandages, salves, and so on) that are easy to come by in settled lands. Giving long-term care counts as light activity for the healer. You cannot give long-term care to yourself.
As OP referenced, Long-Term Care is off the table - specifically called out in the skill as being unusable on yourself.
A creature wounded by stepping on a caltrop moves at one-half normal speed. A successful Heal check removes this movement penalty.
A creature wounded by a spike growth or spike stones spell must succeed on a Reflex save or take injuries that reduce his speed by one-third. Another character can remove this penalty by taking 10 minutes to dress the victim’s injuries and succeeding on a Heal check against the spell’s save DC.
A character can use Heal skill to remove speed penalties from caltrops, but not Spike Growth/Spike Stones (since that specifically calls out "another character".
When treating deadly wounds, you can restore hit points to a damaged creature. Treating deadly wounds restores 1 hit point per level of the creature. If you exceed the DC by 5 or more, add your Wisdom modifier (if positive) to this amount. A creature can only benefit from its deadly wounds being treated within 24 hours of being injured and never more than once per day.
This says nothing about needing someone else to treat you. As such, it's reasonable to allow it.
To treat poison means to tend to a single character who has been poisoned and who is going to take more damage from the poison (or suffer some other effect). Every time the poisoned character makes a saving throw against the poison, you make a Heal check. If your Heal check exceeds the DC of the poison, the character receives a +4 competence bonus on his saving throw against the poison.
As above, nothing in this section says you can't treat yourself.
To treat a disease means to tend to a single diseased character. Every time the diseased character makes a saving throw against disease effects, you make a Heal check. If your Heal check exceeds the DC of the disease, the character receives a +4 competence bonus on his saving throw against the disease.
Again, nothing in this section says you can't treat yourself.
Now, looking at Friendless:
You can make Heal checks on yourself for the purposes of treating deadly wounds, diseases, and poisons.
RAW, this looks like a redundant trait.