Is there any reason to think I won't be able to get a ticket quickly and get a seat on the next train to Kyoto after arriving at KIX about 5 PM on a Monday in August? I'm wondering if that becomes a sort of "comuter nightmare" at that time of day? Or is this something I need to book in advance just to be safe? I'll also need to return early in the morning on Friday (9:50 AM flight out).

No problem. Everyone arrives at the airport and gets on the next train. You could book your return at the same time if you like.

That Haruka "express" train is so not-fast (3,000yen, 1:16) that you may can also consider taking the bus instead (2,500yen, 1:28), at least for the KIX->Kyoto trip where a bit of traffic won't make you miss your flight.

The only reason I can think of is if that August Monday happens to be the 16th this year you may bump into a lot of local residents coming back into Osaka from their Obun travels.

If this happens just follow the stream of people to where they are going and get off in the Kyoto lane,lol. I laugh now but you'll see what I mean if you arrive during the Obun crowd.

Are you utilizing a JR Pass? Where was your final pick of your Kyoto lodgings? If it is nearer the Shoji-dori shopping street then the combo of Nankai and Hankyu railways is the way to go. See here for your rail options. Press the To/From Kyoto button next to the show access information.
http://www.japan-guide.com/e/e2033.html

Aloha!

Staying at Palaceside Hotel. Change at Kyoto Station to Karasuma line to the Marutamachi Station -- exit 2 -- according to the hotel.

I assume there's no point in the pass since I'll just be staying in Kyoto.