Having had a good bit of experience regarding this, often in ways that many others have not, I can probably shed a little light on things for you.
1. If you can excape your body and travel, what makes you think that some negative spirit or force cannot Enter your body while you are
As has been stated here before, you never fully leave your body. Your consiousness/soul/whatever you want to call it never fully leaves your physical
body. Without it, you would die. Your mind is elsewhere, however, and that portion that remains in your body is more or less an empty shell of your
soul. Just enough remains to alert you to potential dangers to your physical body (such as a fire, intruder, etc). At no point is your body empty,
and able to have another entity enter. There are, however, entities that can forcibly enter your body, and these can enter whether you're "in your
body" or not. However, typically, entities such as these won't bother you unless you've somehow called it or upset it. Don't dally with them,
and you'll be fine.
2. I read somewhere that when you leave your body and travel, there is what seems to be a string or cord of some sort attached to your physical
body. And if you feel some strenuious form of stress that the cord breaks and you have a very agonizing jolt back into your physical body, thus
driving you nearly insane.
This is true for all but the most experienced astral travellers. The cord does exist, it is not a tangible thing, but it can be broken. For most
astral travellers, the cord appears as a shimmering silvery cord, sort of like an umbillical cord. This cord with lengthen indefinitely as you travel
astrally, allowing you to travel whereever you wish. It is through this cord that most astral travellers are able to find their way back to their
physical body (which would otherwise be near impossible, given that time and space have no meaning in the astral/spirit world - one step could take
you thousands of years and hundreds of light-years from where you originally stood). As far as the cord breaking, it will not break under its own
strain. It must be broken by an outside force. Some things that can break the cord are: other entities (typically ones that wish you harm),
re-entering the physical world through a door that you didn't originally come through, such as travelling astrally to explore another place on the
physical world (this is only possible if you fully re-enter the physical world - remote viewing is different from physical re-entry - it uses the
parallel aspects of the physical and astral/spiritual to view the physical world through doors back to the physical, without actually stepping through
them. Also, this will not always break the cord, it's just a possibility), and finally, your own will. You are able to sever your own cord. I
would highly advise against it, since severing your cord will often trap you in the astral/spiritual world, and your physical body is as good as dead
at that point.
Those select few that can travel astrally without the cord have a better understanding of the astral/spiritual world, and can therefore navigate back
to their point of origin with ease. This is a skill only learned through many lifetimes if experience - maybe one in 100,000 astral travellers
actually have this ability.
3. What happens if your origional body dies while you are Astral Projecting? My guess is that you will just wonder the earth until.......I
guess Judgement day
If your physical body dies when you are astral, the portion of your soul remaining in your body rejoins your astral form, and you are now a permanent
resident of the spirit world (at least until your next reincarnation).
One of the major misconceptions of astral travel, is that it is a world somewhere in between the world of the living and the world of the dead. This
is not true. The terms "astral plane" and "spirit world" are interchangable. Not everyone you meet astrally will be dead, however. Some have
once lived a mortal life and passed on. Some are other astral travellers, and some are entities that have never had a mortal life, and never will.
Astral travel has as many downsides as it does upsides. For all the potential to learn and grow there, there are also equal potentials for harm. The
best advice I can give you is to do a little reading on the subject (several others in this thread have given books on the subject), learn about the
basics of both astral travel and protection, and be always wary when you do travel astrally.