I am a bit new to churning credit cards and I have a few questions I have yet to get answers on. I tried searching/browsing the forum, but I couldn't find the answers to these. Any help would be much appreciated.

1) I have heard it is good to diversify your credit card issuers, but I don't understand why. I have two CitiBank Cards for the AA miles and a Chase Southwest Card and I'm not sure if there is something I should avoid doing with them? This kind of leads me into my next question.

2) I am interested in using VR cards to pay for my Mortgage and car payment. I keep hearing of stricter limits on buying VR cards such as spending limits, no CC at some retail locations, etc. Who is putting these restrictions on the cards. Is it the stores, or the credit card issuers (specific to me, CitiBank, Chase). If I put $2k each on my CitiBank cards at CVS based on 8 $500 dollar transactions for VR will that put my credit card accounts into jeopardy of being closed, or will they be thrilled I spent so much so fast (obviously hoping it continues...).

Thanks in advance.