There is no limit on how often you are allowed to post observations of a particular species. An observation represents an interaction with a specific individual at a particular time. So if you saw multiple individuals of a species one day when visiting a site, you can upload an observation for each (within limits; maybe don’t post each of the 100 daisies in the meadow separately). You can post new observations of the same individual if you see it again on a different day.
If I understand you correctly, you are using an observation field that links the observation to another observation, correct? You might find that one of the ones that uses taxon lookup is a better fit for what you are trying to do.
If you see multiple insects on a particular plant species on different days, you can post observations for each of the insects and use one of the observation fields that allows for taxon lookup to record the plant being visited (e.g. “Interaction->visited flower of”). As you noted, for the plants it unfortunately isn’t possible to add the same observation field multiple times with different values, but if you use an appropriate field on the observation for the insect, the insect-plant interaction is recorded for both taxa and it is possible to look it up from either the plant or the insect side (though this is not easy to do using iNat’s interface).
These forum threads explain some of the possibilities provided by the observation fields in a bit more detail:
https://forum.inaturalist.org/t/observation-field-for-flowers-and-pollinators/35806/6
https://forum.inaturalist.org/t/site-on-the-predator-prey-relationship/37944/13
https://forum.inaturalist.org/t/listing-of-organisms-associating-with-a-particular-plant/29428/3
To answer your question as to whether noting this information is interesting/useful to researchers: absolutely!
With bees there is also a difference (from the bee point of view, perhaps less so from a pollination perspective) between visiting a flower for nectar (sustenance of the adult bee) and using a flower as a pollen source (for provisioning offspring). Some oligolectic bees will use a broader range of nectar plants; others will mostly use their pollen source as their nectar source also.
I don’t think this nuance can be captured as easily using iNat, since it can be difficult to determine whether a plant is serving as a nectar source or a pollen source based on photos. There is a bee scientist in Germany who has compiled a list of which plants serve as pollen sources for which native bees. I believe he relied, at least in part, on analysis of the pollen found in their nests, which is obviously not feasible for most of us. But even recording which bees visit which plants is still valuable information.