The fact that every object, act and message is inherently indeterminate (lexically and instrumentally) is a direct consequence always have to "read" or "grasp" it in some way - and it always eludes us (ultimately). Now it is of its independent existence: e.g. that it always stands over against us who encounter or produce it, so that we precisely this "standing over against us" of the world, including our constructions in and upon it, that forms the "site" of our mutual communication and understanding: that is, the place where they will be eventually located. By always confronting us while remaining inherently other and indeterminate the things of the world are our continual and fundamental "topic" of conversation (cf. your 2a) around which we organize intentions and coordinate our discourses and practices - where, in effect, we achieve working agreements amongst ourselves that enable us to move on.
Coordination, organization and rhythmicity (and hence sociality) are possible even without mutual understandings about what is going on. To the extent that understanding and coordinated practice become mutually shared and rhythmically synched, we have "participation". But even participation (in which the person-parts become consciously less than the participative whole) does not completely collapse the self with the world that stands over against it, for what you get is loss of ego identity, the body (short of death) remains to stub its toe and part-fall un-cooperatively across the participative space.
Put another way: it is upon the objectification (object-ness) of the world (or the act) that we rely to ground our ability to act and agree in the world. The "over-against-us-ness" of the object provides not just the site of the disagreement, but also the space in which that mutual disagreement or misunderstanding is "absorbed" as it becomes the "topic" of mutual interest and coordination of discussion and practice.
The "topic of interest" then becomes the "site" of the interactional process in which several participants arrive at something new. This new thing is also indeterminate, but it has been arrived at by mutual participation, in which the participants move temporarily through a closer identification with each other in relation to it - whence the object (or situation) is much more than a matter of agreement: it is a place of participative coordination and rhythmicity. It is through this process that all the participants move beyond their original point of indeterminacy (without resolving it) to a new one. The process is an ongoing creative construction of a reality and thus of an "agreed-upon" determinate proposal or resolution, and, of course, and immediate re-introduction of indeterminacy as well. And the process continues.
[After Post-Modernism Conference. Copyright 1997.]