10am or later.

You can start from Ninenzaka (also known as 2 years slope) before heading to Sannenzaka (the 3 years slope, that you've just mentioned)

Weekdays are better choice. Both roads would lead to Kiyomizu-dera. Ninenzaka and Sannenzaka are beautiful in the morning. I prefer to walk on the Ninenzaka steps though, the calm and quiet morning simply enticing and a great way to start the morning. I suggest to start a short walk at 7:00am from Ninenzaka, you will pass by Sannenzaka on the way to Kiyomizu-dera. Once done, you can walk back to Ninenzaka, by then most shops have opened.

Around 1-2 hours...

Higashiyama is a district in eastern Kyoto that contains many historic temples and buildings. Sannenzaka and Ninnenzaka are two streets in particular that have been preserved specially for tourists. They are located near Kiyomizudera, in Higashiyama district.

I recommend to combine Kiyomizu-dera, south Higashiyama (Sannen-zaka and Ninen-zaka slopes), Gion, Pontocho, with Fushimi Inari and Sanjusangendo (the artistic highlight of Kyoto) if you have a full day, and with Sanjusangendo if you have a half day. From Kyoto station, take JR Nara Line to JR Inari to visit Fushimi Inari. Then, take Keihan Railway from its Fushimi Inari (not JR Inari) station to Shichijo(7th street) station and walk 5 min to Sanjusangengo. Then, take a taxi (@1,000 yen) to Kiyomizu-dera or the Keihan again to Kiyomizu-Gojo (5th street) station and walk @15 min to Kiyomizu-dera(temple). Walk Sannen-zaka and Ninen-zaka slopes, see Yasaka Pagoda, Ishibe alley, visit Kodaiji temple and its nice garden, walk through the Yasaka Shrine, and head to Gion Hanamikoji and Gion Shirakawa River areas. If you have time, head to Heian Jingu Shrine. From Gion, you can take the Keihan Railway from Gion-Shijo(4th street) station, or subway Tozai (easy-west) Line from Sanjo-Keihan or Kyoto Shiyakusho Mae (City Hall Front) stations and transfer to subway Karasuma Line at Karasuma-Oike and go back to JR Kyoto station. For the merits of each train system in Kyoto for sightseeing, read the above article carefully.

Sorry, I cannot help you. We were for a very limited time in Japan and just bought shinkansen tickets at the train stations. We never had problem to get seats despite we always purchased "on the edge". The frequency and punctuality is excellent. For example we took the risk to travel directly from Hiroshima to Narita airport to get the returning flight to Canada on the same day and just giving a margin of couple of hours for unexpected events, and everything worked like a precision watch. You may think that shinkansen tickets are expensive but is truly value for money. Good luck in Japan

It would be difficult for a wheelchair user as both streets have steps that you need to use to access the shops. There is shopping that is more accessible on Pontochō which is in the Gion district. Hope that helps!