1. Yes, with payment acquirers.
2. All credit card processors are supported in standard Odoo (which means SaaS too). Yes you read correctly, they are all supported (because they all have an API). In order to support a payment processor, you need to create a payment acquirer for it. You find them in Settings > Payments > Payment Acquirers. In Odoo, a payment acquirer document is the mapping between the payment processor API and Odoo's API. By default, you will find them for Paypal, Ogone, and Adyen. You can create additional ones for any other acquirer, eg. Authorize.net. The easiest is to copy an existing one to see how it's formatted. It requires some technical knowledge. However, unlike Paypal, Ogone, and Adyen, if you just create a payment acquirer, you will only be able to receive payments, not send payments, and not get the success/failure notification of a transaction, and no automatic reconciliation with the invoice (you need to develop a custom module for that). You will have more info at the OpenDays and in the new documentation.
3. You need a journal per credit card. Every time you receive payment, you need to use the corresponding journal. You reconcile manually between your invoice in Odoo and your payments in your processor interface. You handle processor fees with a "banking fees" account and a split in the customer payment window between the amount received and the allocation.
4. This is a vast question.
5. If you don't have to deal with multi-currency, it makes no difference to receive payments from local or international customers. If you are dealing with multi-currency, you can create a new question because that a whole different topic.
6. You don't really care what country the money comes from as long as it ends up in one of your journals, in your currency, and for the full amount of the invoice. a) If it doesn't correspond to one of your journal, then it means that you need to create a new journal to support this new payment method. b) If you receive it in a different currency, then you have to work with global and transaction-specific currency rates as well as currency difference accounts. c) If you don't receive the full amount, then you have to split the total correctly amoung the allocation, the banking fee, and any additional fee you might have, either on the customer payment form or, more granular, in the journal entry.The legal aspect of this question is way to broad to answer here. It entirely depends on your country's legislations.
7. With the standard Paypal module you can request money through direct link or through portal. You can capture the success/failure of the transaction with the IPN. You can transfer the Paypal fee to your customers. You can prepopulate the customer information in the Paypal fields. You can manage installments and subscriptions (custom module). You cannot reconcile invoices automatically (yet) but it is easy to do it manually with the invoice reference on the payment. You cannot refund in Odoo and trigger this action in Paypal automatically.

Hello

Hello many thanks for your support but...

With all due respect

You are answering a non asked question

The question is : how to make the draft so identical to what has been paid

Very simple question

You send a quotation with 2 products but the customer paid for only one

Now as you said i will test the pin option, we will see, but reading python code it seems the so stay into the draft state after the payment...

One question more : where is the documents that warn us we have to write our own reconciliation module between sales and payment? ( to come as i understand ?)

Best regards