Two Questions

I can answer #2. I purchased and used a phone from Eurobuzz last Oct for Italy and France. Worked great, will use it next year when I return to Paris. I just had to remember to add the "+" before the phone numbers when I programmed them into it.

Agree about Eurobuzz -- we used in Paris and Vienna to communicate with apartment caretakers. LIked knowing that there was no way that we would come home to monumental smart-phone data charges (had our smart phones on vacation hold so we used them only for wifi connection and cameras) Great sound quality on Eurobuzz phones.

No, you don't need reservations. (I don't even know if you can make reservations, but not needed).

The museum was good before and is even better now after reopening in 2014 after a 5 million euro renovation.
It now has over 400 Picasso works.

And it has longer hours than many Paris museums.

The Picasso Museum is no longer included with the Paris Museum Pass. It is extremely popular now, so you will want to buy a ticket ahead of time online. When you arrive, you will see two lines (very long ones). One is for folks without tickets, the other one is for timed entry.
http://www.museepicassoparis.fr/en/

Regarding question #2, you might also look at some of the other travel phone firms such as Roam Simple, Cellular Abroad, Telestial or Mobal, just to get a good idea of the price ranges and other features. I've been using one of these for several years, and it's worked well so far.

Eurobuzz, while not as cheap as a local SIM card, is cheaper than any of the companies listed by Ken. Mobal is Eurobuzz's parent company. Both Mobal and Eurobuzz are direct dial systems - you make the call just like you would at home. Some other travel SIM companies have a "ringback" system, where you place the call at a central number, hang up, and wait for the call to ring back when the person answers. I haven't used the other companies, though, so I can't say which do and which don't (it would be described in their fine print). I have had nothing but good experiences with both Mobal and Eurobuzz. You only pay for what you use, and the number is permanent. You can still use the phone and the number for your next trip.

To #1 - I was there. I'm been there very early at the opening hours and had no wait. Slowly gets around that, the museum is reopend.
How the situation will be in September can surely no one predict.

Love EuroBuzz. It works exactly as described on their site. We used two of their phones, too, but of course you could just purchase the SIM...but I like not having to bring my $$$ regular phone out and about. Cobblestones + iPhones = :-(

(Yes, I know many Europeans successfully use their iPhones on the cobblestones all the time, but I'm a lot more tired and clumsy and distracted while sightseeing than I am at home. I may have my iPhone there with me, but I don't want to be pulling it out for texts while floating down the Seine...)

As for only needing it for 8 days...you can either just throw it in a drawer for a future visit, or give/sell it to someone else. DO change the credit card info, though!