Besides Italy the fascist Eastern European nations only gave Germany military support fighting the USSR, without that the Wehrmacht is more strained and weaker when fighting the USSR, probably making it a shorter war. This is despite the fact the border with the USSR is shorter and the Wehrmacht didn't have to invade Yugoslavia(posibly making barbarossa possible earlier) but the spreadout of the German army to the south will put on a lot more strain, even in the first months.
No those fascist nations can not attack Greece and Yugoslavia without going to war with the Allies, they'd be joining Germany in the war if they did that.
Italy not joining will help the allies(and the USSR) greatly, avoiding a long war in North Africa and in the Meds. This will allow them to focus more directly on Germany and later Japan. Of course the danger still exists so the British will not move the Mediteranean fleet away to the Pacifc or anything, but pressure is a lot less, trade routes more secure, industry capacity and naval activity a lot higher. This will help the USSR with lend-lease and put less luftwaffe planes above their lands.