I am assuming most of them would be used as garrison and auxiliaries, but part of it would find itself in the Western Front meat grinder. I don't think the Portuguese were in a much better situation regarding manpower quality, and they sent 55,000 soldiers to the French frontlines.
We agree on this point
I don't know enough to say it would ruin the war effort, but it would sure be at least a big hindrance. Harsher repression of the strike? I'd say so. Depending on how it ends, Anarchism does not get discredited among factory workers as in OTL.
It would since it would spread the narrative of brazilians dying on a foreign war for the brazilian landowning elites to wing some concessions. While there is not a socialist revolution coming (in part due the large unliteracy that makes difficult for worker's to even know who marx is), they can cross their arms and keep fighting the police and the army for months, or things could spirral out of control and ends like the Patagonia tragica in Argentina with death squads helping the government to shot down the rebellious worker's.
The key for more concessions after WWII is a bit more cynical than earlier entry in the war(although it plays a part); it's more men at the frontlines, presence in more frontlines and battles, more casualties. Unfortunately, the only way I can see Brazil getting more than it got is by paying for it in blood, lots of it.
I don't believe on that. There is no way the US is keeping their word on the permanent seat on the security concil and there was no backlash on the brazilian side for the broken trade agreement of the surplus grain so the same might happen on this timeline.