Hi!
I last went to Disney Sea on 12/31/22, so all information is based on that trip and just general knowledge of Disney.
I would pick April 20th because a Thursday tends to be a better day to visit a theme park than a Friday unless one day is supposed to have rain. Bad weather in a theme park tends to mean much shorter lines though a lot of the outdoor entertainment will be affected.
For how to get there, it depends on when the bus from Oshiage runs? I've never stayed there, so I don't know the schedule. The goal though is to be to the park at least 1 hour (or more) before it opens on the day you visit. There will be Japanese people lined up when you arrive. From what I saw on Youtube, Disney has been letting in people earlier than the stated entry time, and you want to be able to take advantage of that, as the earlier in the day, the shorter the waits for rides.
I would 100% recommend downloading and setting up the official Tokyo Disney Resort App on your cell phone. I have it on mine, you will need it if you want to do any Premier Passes.
I am unsure of the Chinese Parks (Hong Kong and Shanghai), but both US Parks, Tokyo Disney and Paris Disneyland got rid of the fast pass system in 2021. All of them now offer a various pay as you go plans. At TDR, you have the option to buy "Premier Passes" for party members. They range in price from 900 yen -2500 yen per person per ride. The 4 rides at Sea that offer Premier Pass access are Soaring, Tower of Terror, Journey to the Center of the Earth, and Toy Story Mania. I think there is also the option to buy it for one of the parades. Right now, for March 17th at almost 11:00 am in Japan, there is at least a 90 minute line for each of those attractions. Soaring and Toy Story Mania have the two longest lines, with both over 120 minutes.
One nice thing about Disney vs. Universal, is you can not buy a Premier Pass until you are physically in the park the day of your visit. (which is also another reason to get to the park as early as possible so you can get in as early as possible.) So you don't have to worry that Premier Passes are already gone for your visit. They won't start being sold until the park is open on April 20th (or the 21st should you visit then.)
I booked my 12/31/22 tickets on 12/12/22, because I had thought tickets were long sold out and there was no way I was going. In general, I've always waited to get tickets until I was in Japan, but I also often stay on property. The website doesn't always work well with foreign credit cards, I had to use Mastercard. However, the Visa I normally use worked fine when I was buying my Premier Passes in Japan as well as on merch/food in the park, it only didn't work when booking my tickets.
Wheelchairs can be rented at Tokyo Disney Sea
https://www.tokyodisneyresort.jp/en/tds/guide/babycar.htmlI have never personally done so, so I can't speak on how it works, you would need to contact Tokyo Disney.
I think in general you're not supposed to hold spots in line at Tokyo Disney, but I also don't use any aids so I am unsure if allowances are made. Though unless it would hurt your pride too much, I would probably start with a wheel chair. I find it is hard not to walk a lot in Tokyo Disney and if you have already been in Japan for a few days, you might have been walking more than you expected. I know I averaged 5 miles a day last trip (except my Disney day) and I was just shopping and trying to take it easy because I hurt my toes. On our Disney day we got to 8 or 9 miles.
Popcorn buckets are fun but after a while they become bulky. We really liked the Ristorante Canaletto. The food is quite good and the view is lovely if you're outside.
I hope this helps!