I think that wording re: check-in has been there for a while, but I have always used F check in area in T2 and T3 (as recently as two weeks ago) , and where available, F check in queue in remote ports and have never been challenged. Also still receive priority tags appropriate to the flight (if no F cabin, just get J tags), Solitaire tags on bag and arrival lounge access at SIN.

The boarding pass will print status as TPXX , where XX depends on the status of the supplementary card holder. Mine has TPEG as I have KF gold status. Otherwise could be TPKF, TPES or TPQP. For other star alliance carriers will show as *G (and VA/UK as partner gold) , picking up Star Gold status as a supplementary card holder.

Your supplementary card holder will also get a second membership card. I get the Solitaire card coinciding with my partners anniversary, but during the year I also get my Gold card on my anniversary. And when I was downgraded to Silver for a couple of months, before I regained Gold, they issued me a Silver Card (but the Solitaire Card was still valid). Also the card shown in the App is the card corresponding to the KF account, not the supplementary status, so if wanting to access Star Alliance lounges and there's a problem with the BP, need to rely on the physical card, not the electronic one (unless of *G in own right).

Oh and to confuse things, I was denied lounge access at ATH flying A3, because it showed me as Silver "in the system", even though my BP showed *G and I had a valid *G card (Solitaire card). I think I worked that my own account had been downgraded to Silver a week earlier, and somehow in the system that overrode the supplementary status. But that's only happened once.