No, you'll not be able to get it "snugged up enough" without the tensioning bolt, its critical for proper adjustment. Don't mess around with it and either replace the tensioner bolt, or have a mechanic replace/adjust it. Loose belts can lead to overheating as those belts also drive the water pump and fan, they need to be at good tension. Do the job correctly and fix the broken part, or be sorry later when something far more expensive (engine), gets destroyed.
There should not be a torx bolt on the lower part of the alternator, thats the wrong bolt if that is whats there.
Should be a normal 17mm hex.
__________________
-diesel is not just a fuel, its a way of life-
'15 GLK250 Bluetec 118k - mine - (OC-123,800)
'17 Metris(VITO!) - 37k - wifes (OC-41k)
'09 Sprinter 3500 Winnebago View - 62k (OC - 67k)
'13 ML350 Bluetec - 95k - dad's (OC-98k)
'01 SL500 - 103k(km) - dad's (OC-110,000km)
'16 E400 4matic Sedan - 148k - Brothers (OC-155k)