How successful are most relationship plans made in the light before birth given all the hazards?

We would say that, on average, at least 75% of planned couplings take place. That is quite a good track record considering the vagaries of human experience and the many, many variables in play influencing a person and their family members. There are usually plans to be in a similar geographic location, although there can be a predetermined arrangement to have a long distance relationship start as a result of a seeming chance meeting during travel which is, in fact, engineered totally to take place so a relationship and a motivation to maintain it can be established at some point despite a geographic separation. This is no problem for the divine realm to surmount. The key is having the lives enough in synchronization that there is a desire, on both ends, for a love relationship to take place, the rest can be pretty straightforward. The fact this is not 100% successful does speak to the uncertainties of life that can intrude and the power, again, of propinquity that will get some people committing to a relationship that arises spontaneously because of proximity and enough affinity in the feelings, on both sides, that things move forward and become more serious and can lead to marriage as well, in which case, none of the pre-life planning actually works out. That is not the end of the world because all of life acts as a teacher, so there is nothing lost, there is nothing wasted in anything that might happen that is spontaneous and unplanned. There will be benefits always, and such things are taken in stride and eventually are seen to be a blessing.