The biggest problem is that you want the plant to thrive in two very different environments (relatively speaking):
Temperature Hot Cold
Moisture Wet Dry
Respiration Liquid Air
Nutrients Bloodborne Soil/compost
Energy Glucose Photosynthesis
Therefore, for this to work, you really need to have a two stage lifespan. Similar to a caterpillar that meta-morphs into a butterfly, your plant will likely have to transition from thriving in one environment to another.
The seed, therefore must be inhaled, consumed, or embedded somehow. Natural processes suggest inhalation or ingestion as the most successful route. While I'd prefer inhalation (close to bloodstream and air, not as harsh as digestive), it appears you've already selected digestive.
So the seed has to not only survive the digestive tract, but the plant has to provide some enticement to being eaten. If it were inhalation, it wouldn't have to entice - it would merely send out spores or microscopic airborne seeds when jostled. The plant, therefore, provides significant satisfying nutrition, so as to get hosts to eat it.
Once consumed, the seed has to pass through the upper digestive tract, but get stuck in the lower digestive tract (lower acidity). This seems difficult, but perhaps it has a germination time of 1-2 hours once the outer acid protective surface is cleaned off. It then starts rooting in several directions, preventing it from moving further down the digestive tract. Small barbed roots that can absorb nutrients from the partially digested food would be best.
At this time it just absorbs energy that will be used during its transition, and builds into a tumor, with longer and longer roots. Eventually it will completely obstruct the digestive tract or the roots will pierce too many blood vessels or veins, and the host will die. If it's necessary that the host doesn't feel pain until near death, then it either synthesizes anesthetic which it emits through its roots, or it doesn't send out roots and instead merely becomes a digestive obstruction.
When the host dies, the process of decomposition sends chemical signals to the seedling and within a few hours it is expending energy sending a tendril upward (any way could be up at this point, but gravity leads the way) seeking light. Once light is obtained, the plant sends out leaves, roots itself more firmly to the host and ground below, and, eventually, flowers and fruits to catch another host for its offspring.
Pulling off the metamorphosis is the tricky part, but since we have animal analogs that do this, it shouldn't be too difficult to explain to the audience.