Okay. there are a couple of questions here, and I'll try to answer them with an explanation and an example.
In Svech's case, he got out of his Russia deal and went to the USHL, and was then drafted in the import draft. He didn't even come to the CHL through a Euro club.
Zadina was outright drafted in the Import draft and signed with the team after terminating his deal in the Czech Republic. He didn't have a valid Czech contract when he came over. Players who can be considered loaned need to be loaned by a team in Europe that they have a valid contract with. Zadina could have done this if his agent structured it that way and Halifax was willing. Certain CHL teams are significantly more desirable to Euros such as Halifax and London, so they can say they won't draft them or take them in that type of deal. In Nylander's case, I feel the team was desperate (as shown by their willingness to hire the dad as a coach), so they were willing to structure the deal in any way. William was just drafted by Toronto, and Sauga isn't a very successful franchise, so I assume their hope was they could bring up attendance through the name association.
Firstly, Brady Tkachuk is a late-birthday. Even if he was drafted from the Knights this year, he could play in the AHL in 2019-20 (for example like Lucas Johansen could this year, while Mikey McLeod couldn't). Now, any player drafted out of a non-CHL league but plays their first post-draft year there can enter the AHL for their 19-year-old year. For example, if Wahlstrom decided to play in the CHL next year after being drafted out of the USHL, could enter the AHL at 19, a clear example of this would be Sonny Milano in the 2015/16 season playing in the AHL despite being 19 and playing in the CHL at 18.
So, basic rules of thumb. any player drafted out of CHL has to play their 18 and 19 year old in the CHL. Unless they are a rare exception like Nylander and Honka ( a new trend agents may pick up on), but still highly uncommon.
If drafted from anywhere else, they can play in the AHL immediately, depending on the contract status they have in their region (this is especially tricky with Russians).
If a player is drafted and goes to the CHL post draft, they are free to leave at any-time.
