Although I am not familiar with any of the tai chi groups that practice in Lumpini, I have seen them practicing. Common sense tells me that they would be practicing during the early morning hours, but one Saturday recently I saw several groups practicing at around 11:00 AM

Nope

Nosotros nos ubicamos cerca del Palacio Real y de una zona con mucho turismo, como es Khao San Road. Acertamos porque está todo, relativamente, cercano, con muy buenas comunicaciones y buenos servicios cercanos durante todo el día.

Entry to park is free but you have to pay for activities, like pedal boats. I think even parking is free on weekends / holidays.

Yes, there are celebrations usually but things might be different from year to year so better to check closer to the time.

Au centre du parc pas besoin de réservation In the center of the park no need for reservation ในใจกลางของอุทยานไม่จำเป็นต้องทำการจอง

今年の２月にルンピニー公園に行った際にミズオオトカゲを二匹みかけました。増えすぎたために捕獲した話も聞きましたが、ちょっと歩けば高確率で見つけられると思います。昼間行ったので朝の屋台の様子はわかりません。ごめんなさい。

No, there aren't any bicycle hire shops there. Cycling is restricted and finishes at 3.00pm

Many years ago there were dogs in cages for sale however on my last trip to the market in July 2019 there weren't any to be found. It does take stamina to spend a day at the market !

I’m in BKK currently and checked the website for Bangkok Symphony, It does not report a concert at the park for 16 December 2018. The most recent concert listed was yesterday, 15 December 2018, at the Thailand Cultural Center. The last appearance by the symhony at Concert In The Park series was listed as March 4, 2018.