1)
Why did he get this name?
1.
Malbim: It was so people will know from this that Ba'al is folly, for Gid'on fights it and denies it and serves Hashem [and it does nothing]. This was preparation for the salvation, which could not come as long as they believed in Ba'al.
2)
Why does it say "Yarev Bo"? Verse 31 said "Yarev Lo"!
1.
Rashi: Quarreling with someone (in this case, Gid'on) is Yarev Im, Es or Bo, like "va'Yarev b'Lavan" (Bereishis 31:36). Yarev Lo is to quarrel on behalf of him (Ba'al).
