Newbie question about getting a loan

[Koshu Takatsuji]

Hi all, 

I'm currently looking to get a conventional loan to buy a SFR, but it's my first time doing this so there were a couple of questions I had.

1. If I fill out too many loan applications (to shop around) and they pull my credit, doesn't my credit score drop significantly?

2. If i get pre-approved, do I get to lock in my rates, or am I not able to lock in my rates until I make an offer on a property? 

2b. If i do get to lock in my rates, what if i end up choosing a property after the lock-in period? Do I have to submit another loan application for another pre-approval?

3. If I am getting a loan with my spouse, whose credit score is used? Is it the better one? The worse one? An average?