In 4.5-mat, I would probably sit like nijo with the guest sitting next to the temaedatami. He has no issues with that.
If not sitting there, he would put the guest where we usually put them for okeiko (first half of the kyakudatami mat).
In both situations, the host should join for koicha.
In the nijo-like sitting arrangement, host could join them on the kinindatami, or host could sit in the first part of the kyakudatami.
If shokyaku is sitting in the first part of kyakudatami, host should sit in the kagetsu kariza position.
Teishu should tell kyaku at the opening (hishaku drop) sorei that they will join them for koicha. Then, when tea is served and kyaku brings it to their seat, they should leave it herisoto (so it’s vibes-wise between them and the host, versus bringing it heriuchi and left), then sorei with the host.
After sorei, teishu asks ofukukagen, then comes and joins.