Given the shine on page 6, I think the large forehead is very much intentional and he seems to be completely unapologetical about it.
Also, Jyungo, calm down, it's a pretty rough and simple drawing.
And as far as I'm aware, seven head beast is more of a force of expression thing, though it is used pretty much the same way as it was used in this chapter: to denote that something is not as complicated as it could be / previously expected to be.
EX: "Yeah, so playing te guitar is a bit tricky, but it ain't no seven headed beast"
I cannot point out the origin of the expression, but given how christianity played a big part on Brazil's formation, and this is only a guess so take it with a boulder-sized grain of salt, I would think that it probably has a relation to the seven headed beast of the bible, maybe some indians used it as an expression after a priest tried explaining the beast to them, and it just stuck.
There's also a brazillian movie on Besouro. The only thing I recall about it is not being very impressed, but hey, if anyone else care's for it. https://en.wikipedia.org/wiki/The_Assailant