Madrid Trip in October

For your first night, I'd suggest starting at Plaza de Santa Ana and then heading east from there (Calle de Echegaray, Calle de Ventura de la Vega, etc.). I love that area at night and it has lots of options for tapas and more traditional sit down places. Another good option is Calle de la Cava Baja. Both areas are very central and can be pretty crowded.

As for Toledo train tickets, I think most people still buy them as walk up, but it is a very popular day trip route so buying in advance makes sense to me. The last time we were in Madrid we bought them at the Atocha station and had to wait for another train later as the next train was full. It wasn't a big deal for us but if you are on a schedule it may set you back.

The most popular area for tapas for visitors is probably La Latina, where there is a string of restaurants (dozens upon dozens) lining Calle Cava Baja. If you don't mind eating standing up at the bar, you can go anywhere, at any time. If you want to sit at least for a time, I would book a bar seat at one restaurant, (if they do not take reservations, appear at opening time) and have a couple of tapas before moving on, taking your chances on finding space... Casa Lucas is excellent and you can book ahead:

https://casalucas.es/english/

If you want. much less tourist-oriented experience, I would head for the barrio Retiro, east of Retiro park, an area popular with locals where there are many restaurants offering tapas..again, I would book a seat at the bar or, barring that, arrive at opening time.
Here is a map of the neighborhood, with restaurants/bars marked, and comments in English:
https://zenderflea.wixsite.com/spainmiryoku/post/madrid-east-of-retiro-park-the-gastronomy-mile-the-most-popular-tapas-area-in-madrid

You could begin at Tqberna Laredo; they take reservations, and move on to places like Arzabal Retiro, La Catapa, etc... YOu'd need to book Laredo a week or more ahead of time, online, or have your hotel phone them..

Taberna Laredo has great seafood, among many other choices:
https://tabernalaredo.com/carta/

1. No other museums really require booking in advance normally.

2. You can't really go wrong as far as neighborhoods go. I always like to get tapas in the evening across the Retiro, in Salamanca or Ibiza, because you get that nice walk home through the park, but that's a bit further from the center so maybe not the move for a first time tourist. I would just avoid the Plaza Mayor/Gran Via, the most touristy areas tend to have mediocre food at high prices.

3. I went to El Trébol the one time I visited and it was a solid 8/10.

4. Not really sure, personally, i would book in advance because it's a high speed train with reserved seating, especially could fill up for morning times for Madrid tourists.

5. Recommended tapas

Malasaña & Chueca:

• Pez Tortilla
• Casa Camacho
• Mercado de San Ildefonso, Mercado San Antón, Mercado Barceló, & Mercado de los Mostenses
• La Pescadería
• Josefita Bar

Salamanca:

• Manero Madrid
• Mercado de la paz, esp. Casa Dani

Central East of Sol /west of Retiro:

• Las Bravas
• La Casa del Abuelo
• Fatigas del querer
• Mercado Antón Martín

Lavapies:

• Bar Melo's
• Bar Tribuetxe
• Mercado de San Fernando
• La Fisna Wine Bar
• La Canibal

South of Plaza Mayor:

• Casa Revuelta
• La Concha
• Casa Lucas

Ibiza:

• La Catapa
• La Castela