Minecraft – How to let people join the minecraft server from diffrent versions

Your tests seem perfectly valid. If the game generates the biomes properly with seeds that have those biomes, then you're simply suffering from bad luck or you aren't looking around enough to know that your desired biomes are not available. They must be there somewhere, but some of the newer biomes are frustratingly rare or commonly distant from spawn.

As was mentioned in the comments, to more easily verify the convenient placement of a given biome in a given seed is to test it in AMIDST. That way you won't have to fly around in creative mode to tell what the biome layout will be.

This is a problem I also have when trying to play LAN with a mac and a windows 10 computer. When the mac hosts, everything works out wonderfully. When the windows 10 pc hosts, the mac can't find the LAN world. Here's my solution.

First, host the LAN server on the windows 10 computer. There should be a port given in the chat. Remember it. Then left click on the network icon in the lower right corner of the screen, and press "Network settings". On the screen that pops up, press "Advanced options". Scroll down through the properties until you get to the IPv4 address. There should be four numbers separated by periods, similar to this: 192.189.54.78. Remember them.

Now open up your Mac's minecraft. Go to multiplayer, then direct connect. Fill in the address with the IP first, then the port, like this: 192.168.54.78:56234

Then press Join Server, and you're good to go!