ok...lets see...
venice at most can bother the oe with regards to the straights. also, unless minting like nuts a venice player will have huge disconfort in raising troops(25 ducats for 1k cavalery). unless absolute luck in the random battles outcome an invasion by anatolia would be very upractical for venice earlly on, not to mentione her loosing time in gaining the italian minors before france and spain do so as well
moving on, mameluks CAN invade oe but their troops will soon run out of steam even if their initial invasion would be successful( a human would not play as the ai in the sense that would not just engage in any fights mameluks would like them to and the opposite). the oe will simplly win any invasion due to its superior militarry sliders and manpower( mameluks has VERY low manpower due to wrong culture/no cores. of course the OE might need to take a loan or 2 in the worst case scenario, but nothing it can not handle.
oe only needs to dow byz right at start and annex( there is absolutelly nopthing mameluks or venice can do about it this earlly...). the cot event triggers right after regardless if mameluks keeps ottos at war or not...just the capital move is requiered for oe to be at peace...
the above scenario(wich usually happens if all players at the same "level" of eu2 mp) brings the oe -french alliance to forefront and it gives it a real purpose from start to finish IF the players follow even remotelly the historical course. so it wont be just austria vs france or ottos; will be france/oe vs mameluks/venice vs austria/spain(maybe).