On my many RTWs I have never had any of the problems you describe with OPEN tickets, with one exception when AY insisted on stickering an OPEN coupon at HEL, but this took a few seconds and incurred no charge.

Putting a date on the ticket cannot affect the taxes - the only thing that does is whether you had it ticketed as Transit or Stopover. It is perfectly possible to specify a Transit on an OPEN-dated coupon in order to minimise taxes. But if you subsequently wish to change a Transit to a Stopover then that requires a ticket reissue, which incurs the USD75 reissue fee. (Changing a stopover to a transit would also, so it is rarely justified since the tax saving will almost always be less than the reissue fee.)