In california you can file your own LLC or S-corp yourself. The fees are around $200. You do not need a lawyer. Sites like legalzoom do not do much but fill out forms available from the secretary of state's office. The annual fees in california are $800. This is a minimum tax the state charges.
As for merchant accounts Braintree is a good choice.
If you have multiple products, it makes sense to have just one merchant account.
This is how i would do it.
Form an LLC for your Parent Company.
Sign up for a merchant account for mywebventures.com
Next create your sites.
Make sure all sites state that they are owned by mywebventures.com
Last, i would regiser a 4th domain name... for customer statements.
This could be called: webventurespayment.com
This is what i would ask briantree to print on your customers statments.
I would make a simple 1 page site that when your customers visit lists all your projects, so that they can realize what product they purchased and to keep down disputes.
Also, during the credit card processing, remind your vistors that it will show up on their bill as webventurespayment.com
The reason for putting all your products under a merchant account are:
1. Lower fees in the long run.
2. Only one monthly fee
3. Easier to manage
4. Easier to build payment history and eventually qualify for lower interchange plus pricing.
A side note on brain tree. Those guys are awesome but they typically lock you into their gateway. I would avoid this because it means you use them for your merchant account and gateway.
I am most comfortable using a merchant account + authorize.net gateway. My apps use authorize.net. And if i switch merchant accounts, all i do is make sure i keep the same gateway username and password for it, and nothing else changes. This lets you shop for lower fees and rates as your transaction volume grows.
Last, some merchants allow for multiple descriptors (what shows up on statements) and merchant sub accounts, but they are hard to find.
I would give a call to FlagShipMerchantServices.com ask for Kirk McWorther. I did a lot of shopping for merchant accounts and cannot give enough praise to these guys. Amazing service, honest, and work with you as your businesses grow. Let them know who sent ya! Best of luck.