Hello Jon, and welcome to the Cove!
1) Given we have CE and FCC approval (and their is no external power), is this likely to have a positive impact on the time taken to achieve InMetro approval?
Not necessarily. Inmetro certification can use test reports from abroad, and if they are acceptable, they will have a positive impact on the time (because you do not have to retest), but CE and FCC approval means nothing in principle.
2) is it advisable/necesary to have the local (Brazilian) manufacturer to apply for InMetro approval?
It's a requirement that the ANVISA registration holder in Brazil (whoever it is) apply for the Inmetro certification.
3) is it advisable to have a local legal representation to process the Anvisa application so we don't have a problem if we add additional distributors?
You can use consulting services that act as BRG (Brazilian Registration Holders). I myself am against this, because I think they are too risky from a business standpoint, but if it make sense to your business, it's an option.