For simplicity sake when i mention a "lender" it will be an individual lending cash and receiving a interest rate in return.
When I mention a "partner" it will be a individual receiving a percentage of the deal. (you will split profits with)
It is not necessarily very hard to borrow from a private "lender", but it is a heavily regulated arena. There are federal SEC regulations as well as sate regulations, each state is slightly different. You definitely want to research this before pursuing it. I would say a large portion of investors are not SEC compliant, not because they are shaddy or don't care, but simply because they don't know. check out this thread (don't skip the 2nd page)
It is a bit simpler to borrow from a "partner", while definitely not ideal in all situations. There are numerous posts about partnering on deals, use the search function in the upper right hand corner and search for "partnerships" or "JV"
A private "lender" would not just send the cash to your bank account. They would send it to the tittle company or closing attorney at the time of closing.
If a deal goes bad with a "lender" and money is lost you take the loss. If the deal is so bad that the property is not worth what you borrowed and the "lender" forecloses on you, then they potentially take a loss as well. If you loose money on a deal you did with a "partner" it would depend on your partnership agreement.
Protections in place for yourself would be, know what your doing, use an attorney, use disclosure statements (required for "lenders"). Protections for "lender" is being on title as lien holder, title insurance, hazard insurance, consulting an attorney. Protections for "partner" should be clear in partnership agreement, partnership agreement should be made by an experienced attorney, and others will vary based on deal structure.
Splits of profit between different "partners" can vary greatly for a number of reasons. 50/50 is common for a "partner" funding purchase+rehab while you do all the work. It really depends on experience. In your case of "partner" A funding purchase and "partner" B funding rehab you could do something like you get your 50% for running the rehab and they split the other 50% equivalent to how much was contributed. (if rehab costs = purchase price they would both get 25%)
When you are evaluating your deal you should be accounting for your listing agents commissions, if the deal is to thin to pay an agent you probably should not be doing the deal.
In your last comment where you have investor A fund purchase, B fund and do rehab, and you in the middle (pretty much just finding the deal), it would be much more practical to wholesale the deal to investor B in the first place.