FYI Borghese Gallery booked up May 3

That's because it's a first Sunday of the month when entrance is free. While reservations still must be made, available time slots fill up very, very quickly.

I missed this. I do not see anywhere in RS Rome 2014 or the update that it is free 1st Sunday. I would have planned around this.

I do not see anywhere in RS Rome 2014 or the update that it is free 1st Sunday

That's because free first Sundays at Italian state museums weren't initiated until July of 2014 so the info was too recent to make the book. Here's one of the notices from local intelligence last summer:

http://www.buzzinrome.com/2014/06/23/revolution-in-romes-landmarks-rules-fees-and-opening-hours-change-from-july-1/

At the same time, the Cultural Ministry changed the photography rules and entry for EU citizens over 65 (no longer free). The Borghese - as well as some others - hasn't yet updated their website to reflect all of these changes. In any case, it's a VERY popular museum so booking as far in advance as the website will allow (60 days, I believe?) is always a good idea for high and shoulder seasons.

In http://www.ricksteves.com/europe/italy/guidebook-updates under "Across Italy" it reads

For books printed before September 2014, the following may apply:

•In all Italian state-run museums, photos are now allowed (without flash), including at the Accademia and Uffizi Gallery in >Florence, and at the Accademia in Venice (this change, however, may be temporary).
•In all Italian state-run museums, admission is now free on the first Sunday of each month.

When we visited Italy two years ago, I forgot to purchase Borghese tickets beforehand and by the time I remembered, the website visited them as sold out. We decided to stop by anyway (based on RS suggestion) and when I showed the ticket counter our Roma Card, they give us tickets (5 in total).

I would give walk up a shot.