My family of 4 went in October of 2017, and hands down, I think Halloween is just done really well in Disneyland parks in Asia in general. While we were there, it was very chill and lines were not long at all for one of their busiest times of the year. So I think you'll be fine in April. Compared to Disneyland in California; 30-60 minutes waits are nothing.
As amazing as I love our trip, what I haven't seen people comment is HK is the smallest of all the overseas Disneylands. It doesn't have a secondary theme park near it like everyone else. You can easily spend half a day going through all the rides, and the other half enjoying shows and the little details. A 2nd day would be to go back on favorites and shop/dine. We went for a 2 days at the beginning of the trip, and because it's so cheap to go, we went for 6 hours before we flew out on our last day.
To answer your questions:
1. Six days is a lot to spend on Lantau Island, HK is so friendly to get around, stay more centralized and get around! It takes about 45 mins - 1 hr to get on and off the island and there's just so much more to see and eat off Lantau. Plus, all the delicious food you are 60-75% cheaper outside of Disney. One thing I would have loved to do was may resort hop at 2 or more of the resorts, they're experiences themselves. So if you're into hotel hopping within Disney, stay on the island. But you'll find, you may not go into the Park as often as you think.
2. April can be a bit rainy, but it's not as hot as the Summer. Even on the worse day of crowds, you're not going to wait more than about 60 minutes for any rides, and Fast Pass the bigger ones help. HK is more of an afternoon to evening crowd compared to most parks as they're a working culture. So if you go first thing in the morning through about 3p, you're good. After 3p, because the park has specials for after hours, it gets a wee bit crowded.
3. Definitely get an Octopus card, load it up like a debit card cause you're going to find that you can use it at markets and cafes as well as taking trains. Don't hesitate to ask for directions when you get on the train stations versus trying to fumble around. The reason their transit works well is that people follow lines and directions well to maintain order. Train station agents are super helpful, English is spoken everywhere. Have the transit website saved on your phone, it's super detailed and can help plan a route in no time. YouTube has one amazing videos on short things to do in HK that will be helpful too.
Disney tips!
- Create a cool little book for stickers! In HK Disneyland, Cast Members hand out stickers of every theme and variation. During Halloween, we got so many cool Halloween themed and villains you just don't see ever! Kids and adults can ask for stickers and it's just a cool free thing to collect. I loved it more than the button. I made my daughter a sticker book of the different lands, and cast members just loved helping her complete sets of stickers.
- Food in the hotels can be comparable to US markeup prices, I really enjoyed eating in the park before heading back to the hotel. Again, better yet, explore outside of Disney for better food. Ginger Chicken at Disney is like $69HK at Disney, then of Lantau Island, it's like $8HK.
- Mystic Manor. Hands down, my favorite ride anywhere Disney. Ride it often!
Have fun!