I recently switched colors from a flouro yellow PLA to a red PLA, both from the same vendor. I’m seeing fine strings between parts and internally in the parts. Any idea on what’s happening and how to correct for it? Is this normal tweaking needed when you swap spools, or only likely when changing colors or vendors?
That is very very minimal. If you really want to try and get it a bit better, you could try and speed up retraction or even the retraction accel (the downside is greater chance of mouse bite on high retract parts). Or ever so slightly increase the retract distance.
Temps can play a part so can cooling fan, but I’m guessing you tried those.
I’m leaning toward temps and trying to print too may parts at once. That one had just the two pieces, I was seeing much weirder effects when I tried printing eight pieces (three different, two each of two and four of the third). My thought was each piece was cooling too much before the print head got back for the next layer.
Yea, that’s a hard one to fix. I’m happy when I get my retract setting to look like your current prints! Petg is worse since it’s a really sticky plastic when molten.
Looks like the bulk of the spider webs have cleared up. 
Though I printed a bunch of thumbwheels this morning and one ended up with a blob on it’s side about half way up.
These are due to the retractation, either retractation speed or length. You can fix it by one of these 3 means:
-Increase retractation length
-Increase retractation speed/accel
-Decrease print head temp
You have very little, so no big deal here. You can easily clean them up using a lighter.
I think the main issue you have to work on is warping, considering the last picture you posted 
The warping you see on the one piece was a bed height issue. That first layer didn’t stick due to that corner being a little low. I replaced the leveling with longer screws and longer springs and reset my X axis to be parallel with the printer frame base. That should cover it I think.