WOW...This is a tough one...OK, first of all, it is perfectly acceptable to host your own engagement party. Couples do this all the time. Now, I have no idea what the long story is, as long stories tend to be pretty complicated and stressful. I can only imagine that you and your boyfriend must be feeling that stress as well. The most important thing I can say is to not let the family stress put a strain in your personal relationship with each other. You two are going to spend the rest of your lives together, and it's super important to learn how to get through tough spots together AS A TEAM. Learning how to do this now will set the tone in your relationship when other bumps in the road come along. Support each other first!!!!
So, when you do indeed get engaged and host the party, remember that YOUR (meaning you and your new fiance) emotional stability will set the tone of the party. Make sure you two do something RELAXING together the day before or the day of the party. You will need to focus on each other. Remember, this party is to CELEBRATE your relationship and your HAPPINESS first and foremost. Having the families meet should be a secondary objective. KEEP THE FOCUS ON THE CELEBRATION. It may be a good idea to also invite as many of your loving and supportive friends as possible to help you set the tone as well as to help you recenter yourself if the families get a little heated. You and your fiance may not be able to keep the families in check because you'll be outnumbered if you don't have any friends in your corner that night to back you up.
Your families need to see how well you treat each other, how well you work together as a team, and most importantly, how much you love each other. They are probably upset because they have a different idea in their minds of what they think is best for you because they love you. But, when it comes to love, there is no right or wrong answer. It may take some time for them to get used to the idea that what they have in mind for you is different than what YOU want with your life. This is not going to be an easy transition for you or them, so you will need to be the "rock". Be honest with your family and tell them that this is a healthy relationship, show them with your actions, and explain to them that you understand why their upset and they now need to understand your decision and why it's so important to you as well.
Good luck! I'll be thinking of you!