As has already been pointed out, not all arguidos are suspects but suspects are usually designated arguidos before being questioned. The terms arguido and suspect are not mutually exclusive.
The Portuguese refer to arguidos as defendants so that is good enough for me.
To make matters more complicated, in the Article on deadlines, the PT code mentions both "suspeitos" and "arguidos". It doesn't clarify the distinction. The only potential one that I can think of is if a "suspeito" is under covert surveillance prior to being made arguido.
As pointed out on a different thread, the notion of founded suspicion was introduced in Sept 2007, as well as the right to be informed of the facts that led to it.
Waving a piece of paper in the air insinuating damning forensic evidence was no longer an option.
On your second point, arguido doesn't mean defendant in the UK / US sense of a person being prosecuted / sued in a court of law - although that would be the term if ever that were to happen.
Don't forget that there are several phases in the Portuguese criminal process.