@ebbe the Axis could still reinforce the Black Sea by using Danube River. Though not sure if it was wide enough to transport any kind of warships. Also not sure if all Axis Submarines were be able to sneake the Black Sea via staits.
Western Turkey was way more populated and industrialized than the Eastern one. If there will be partisan activity in Eastern Turkey, i can see no reason why wouldn't it be in the Western Turkey as well.
Realistically I don't think Spain and Portugal would declare war on Russia just because Finland and N.Norway were occupied. If they are completely occupied then it means the Russians already took back all of its original territories since Finnish front was the least important front for Russians.
It would definitely worsened the relations between Russia and Sweden but still not enough casus belli plus nations wouldn't want to join wars on loser side.
I think there is no scenario which Portugal joins the Axis.
Spain could have joined the Axis only if she was confident that Britain is already about to lose the war and significant German assist will be ensured. Spanish entry into the war could led Portugese entry into the war as well.
Spanish entry also could led conflicts between Spain and Vichy France over the control of African colonies.
Also iirc both Spain and the Soviets (until the Barbarossa) harbored the u-boats.
I have no knowlegde about Mexico and Brazil in WWII. Maybe they would delay their entance on the war if Germany decided not operate u-boats near of these countries?
The Italian situation is complicated maybe @Cernel could correct me.
I can't understand why Mussolini was so unguarded when it became appear that there was no way to stop upcoming Allies invasion of Italy without heavy German assistance which they already refused to fight for controlling Southern Italy and start to suspect Italian loyalty to Axis. At this point, Germans were interested in controlling Northern and Central Italy whatever Mussolini wanted or not.
It is not clear for me would Mussolini still be out voted and arrested even if Allies failed to invade Italy? Or couldn't Mussolini be so unguarded and prevent Italy switching side?
There was already heavy German presence in Italy and Italian occupied territories when the armstice signed and the Germans just disarm the Italian army then rearmed when Italian Social Republic created.
Turning Italy to Allies after losing Sicily and S.Italy is realistic but I don't think it was an unavoidable outcome. Nations may surrender without losing any land or not surrender even after losing %95 of lands. It really depends on parameters.
I think it would be more realistic turning Italy Allies with a possibility rather than automatically turning it Allies when southern one get occupied by the Allies.