Offer for a pre-PhD visit, how to handle the situation with other PhD applications in the queue?
In general, I think so long as you and the PI in question are forthright about your mutual expectations, there should be very few problems.

If the PI who is offering you the position is planning on having you become a PhD student, then that should be made clear. Similarly, if you'd like to consider other options, then you should let your potential advisor know that a "competition" will be taking place.

However, since you said you'd be writing a PhD proposal, it's not clear that you'd automatically have a guaranteed offer for a position at the end of this position if successful. This should also be cleared up before you make a final decision.