When you first talk to Hjollder in Koldehar he offers to take you to HoW expansion, but he also says he can bring you back once he is able to rest for a day. Is it in fact possible to return freely to IWD1 from HoW, or is this a one-way trip?

You cannot move freely between the two. You are returned to IWD at the conclusion of HoW, so it's not a one-way trip, however. The ideal way to complete the expansions is just before going up the stairs to Poquelin (the rest of IWD becomes too easy with all of the equipment and XP from the expansions). Once you have the items required to ascend the stairs, go back to Kuldahar, place those items in a chest, and speak to Hjolder; travel to HoW. Complete TotLM before HoW: don't cross the Sea of Moving Ice! Instead, go to the tavern in Lonelywood to begin TotLM, then once that is complete, carry on with HoW. You will be dumped back in IWD proper at the conclusion of HoW.

K thanks. That's the last time I ever trust a Barbarian :)

You can get back to Kuldahar at other times as well. If memory serves pretty much whenever you meet Hjollder again at the end of a chapter he will offer to take you back - if you do he will then wait in his house until you want to return.

I have been to and fro a few times, it was very hard the first time as I agreed to go way too early in the IWD story.