Sour Mash starter question

So, I've been reading up on sour mashing and want to start with something small like a sour-stout...along the lines of a Guinness. I plan to use a 2L flask as my mash vessel with a heating pad connected to a temperature regulator I built.
My questions are as follows...

1, Do you need to do a traditional sacc. rest, then cool, pitch fresh malt. What is provided from the sacc. rest that is needed besides sugars? Can I just heat up some table sugar in water and add some un-mashed grain?

2, If not, if it's better to sacc rest in order to generate a more complex range of flavors (otherwise I could just add Lactic Acid to the finished beer, right?), would making a 2L batch of starter wort with extract provide the same "food" as doing a sacc. rest?

It seems to me that making a quick extract starter batch, tossing in some grain, filling to the top, plastic wrapping, and finally maintaining a warm temp is the easier/quicker way to go, rather then going through a full sacc. rest.

If I was planning on doing a Berlinner, I could see going through the whole process for a full grain bill sour mash, but for adding just a hint of sourness to a mild stout, it just seems like too much work.

Thanks in advance for you responses...

Yes, you can just use extract, if it makes it easier for you.

I have made a couple of Berliners using just wheat extract, and Lactobacillus Acidophillus (from crushed probiotic tablets). Kept at 100 F for a week before adding yeast.

Sours up fine and makes a great refreshing beer.

I use the probiotic tablets because they are cheap (versus WLP or Wyeast products), and do not produce any of the 'vomit' smells/products you can get with grain. However, they do not work if there are any hops in the wort.