While you pass a qv variable to SQL engine, the evaluation of variable still gets done by QV and the resultant is sent to SQL engine. That means, your variable definition:
Let vLoadFrom = timestamp(If(Weekday($(vLAST_LOAD_TIME)) >= 5, WeekStart($(vLAST_LOAD_TIME)), $(vLAST_LOAD_TIME)), 'DD.MM.YYYY HH24:MI:SS');
is supposed to be evaluated by QV. But the time stamp format 'DD.MM.YYYY HH24:MI:SS' is unknown to QV. So you don't get the desired output.
Hope this helps.