Do you have Cougar White (Super Smooth) 100# but in 12x18inches? If so is they're a way for me to order a pack of 200 sheets?

At the link below you can use the dropdowns to select 100# cover 12x18 cardstock. It is available in packs of 150.
https://www.cutcardstock.com/collections/premium-cougar-card-stock/products/discount-white-card-stock

Does this card stock crack at the seam when folded for a greeting card? Will it hold up to scoring and folding for cards?

It should not crack when folded, if it has been scored before folding.  The heavier the stock gets, the higher chance of having it crack.

Does Cougar Super Smooth White card stock 100# cover come in A7 folded cards (sent flat 7x10) ? I see I can order the 8.5 x11 100# cover (which I have been scoring & cutting). What would the price be if I ordered 100 or 250 ??? Or would it be less expensive to just continue scoring & cutting myself? Thank you;

Here is a link to the Cougar White A7 Folders in bulk packaging in various weights https://www.cutcardstock.com/collections/blankfolded5x7invitations/products/bulk-folded-5x7-discount-card-stock?variant=39306416226398
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Does this come as an A2 folded card (100#)?

Yes.  Here is the link to the folded cards. https://www.cutcardstock.com/collections/blank-a2-folded-notecards/products/blank-white-a2-folded-card-stock

Hi! Is it possible to have all edges of cardstock being custom cut to be cut with deckle edges? Thanks so much, Mary

We do not have the ability to do custom deckle edges.