#1a: A <slider> is not a stimulus and thus cannot be displayed via a <trial>'s /stimulusframes. You need to put the <slider> on a <surveypage> (which is in itself a special kind of <trial>) by using its /questions attribute.
#1b: Run the <surveypage> via a <block> at the 11th position. From that <block> /branch to either <block task1> or <block task2> depending on the <slider> response given. A simplified example summing up 1a and 1b:
<expt>
/ blocks = [1=task1block; 2=task2block; 3-6=choiceblock]
</expt>
<block task1block>
/ trials = [1=mytrial]
</block>
<block task2block>
/ trials = [1=mytrial]
</block>
<trial mytrial>
/ stimulusframes = [1=mytext]
/ validresponse = (57)
</trial>
<text mytext>
/ items = ("<%script.currentblock%>")
</text>
<block choiceblock>
/ trials = [1=choicepage]
/ branch = [if(slider.choicequestion.response=="1") block.task1block]
/ branch = [if(slider.choicequestion.response=="2") block.task2block]
</block>
<surveypage choicepage>
/ questions = [1=choicequestion]
</surveypage>
<slider choicequestion>
/ labels = ("Do Task 1", "Do Task 2")
/ range = (1,2)
</slider>
#2: You can specify a /timeout attribute at the <block>-level, which will determine the maximum allowable time for the block. If the block runs out of trials prior to the timeout, it will terminate. Thus you need to make sure the block runs enough trials.
#3: For simple feedback, you can use the <block> element's /blockfeedback attribute, details for which you can find in the documentation. For more elaborate feedback, set up <values>, <expressions>, etc. to calculate your performance statistics of choice and display the results at the end of the block either via an instructions <page> (/postinstructions) or standard <text>, <trial> etc. elements (see e.g.
http://www.millisecond.com/forums/FindPost14025.aspx ).