Official Baseball Rule 5.09(b)(1) states that a runner is out when "they run more than three feet away from their base path to avoid being tagged unless their action is to avoid interference with a fielder fielding a batted ball. A runner’s base path is established when the tag attempt occurs and is a straight line from the runner to the base they are attempting to reach safely."
Remember, the base path is established only when the tag attempt begins, meaning that while the ball is still in flight, no such line can be drawn.
Meanwhile, we have the issue of potential obstruction. OBR's Definition of Terms states, "OBSTRUCTION is the act of a fielder who, while not in possession of the ball and not in the act of fielding the ball, impedes the progress of any runner" while OBR 6.01(h)(1) pertaining to Type 1 Obstruction specifies that when obstruction occurs while a play is being made on the obstructed runner, the runner is awarded the next base ("The obstructed runner shall be awarded at least one base beyond the base they had last legally touched before the obstruction").
In a similar token to when to begin a base path, we need to address when to begin considering a fielder to be "in the act of fielding," while noting the pre-season point of emphasis declaring that a fielder who blocks the runner's path should be called for obstruction if they do not legitimately need to occupy that space to field the baseball.
By virtue of fielder Arenado running at Bellinger—and into his path to third base—prior to teammate Masyn Winn even releasing the baseball, we know Arenado cannot be considered in the act of fielding when he initially impedes Bellinger and this play, thus, becomes a candidate for Obstruction Type 1 (Type A), effectively cancelling the later base path issue since play is dead at the moment of Obstruction 1 (had it been OBS 2, then "nullify the act" would get involved after the play's natural conclusion).
Video as follows:
