To be clear, FTC elims (for regionals/districts, in my experience) are typically four alliances of three teams. In the brackets, alliances play best-of-3, in which each of the three robots on an alliance must appear at least once (i.e., all 3 teams are guaranteed to play).
Defense in FTC (in my experience: refereeing recent years) is very limited. This is a simple consequence of two-robot alliances, because with two-robot alliances it is even more unlikely that you can prevent more points than you can score (similar logic to if you knew your 3rd bot was not going to move in 2019 FRC, you would most likely use both the moving robots on offense). Now that I think about it, this is another downside (if you like the existence of defense) to two-bot alliances.