I know there is a difference in cable pull between the brifters and derailleur, but is the spacing still the same? Wondering if I can put a Tiagra 10 speed cassette on my bike which has 105 10 speed.

Yes, same spacing, will work fine.

Yes, I have the same arrangement on two bikes and it works perfectly.

Yes spacing will be the same for 10 speed shimano. It is when you go to Campy that things change a bit. The 4700 shifters and derailleurs are technically only compatible with themselves (though are purported to be 11 speed cable pull meaning maybe they could work with 11 speed Shimano) but the cassettes and other bits should work just fine. I had a mechanic recently who tried to use 4700 derailleur on 4600 shifters and couldn't get it working right luckily I was there to save the day and we ordered the customer an upgraded derailleur that will actually work because it is 10 speed pull. All is good.

It was an odd choice on Shimano's part to make 4700 only work with 4700 officially as it has been screwy for the folks who own 4600 and need new parts but they are probably planning for the future as I am sure they knew 12 speed is coming so they are probably trying to keep that derailleur for 11 speed Tiagra which may happen in a next few years.

Thanks all! Going to get that cheap Tiagra cassette to find out if my bike will take a 30t since I am climbing much more slowly than I did 10 years and 55lb ago.