The first thing I would say about modal verbs is that their usage extends over many different contexts and so any general rules are bound to fail when taken out of context.
Outline
- Use of 'will' and 'would' to express belief
- Other uses
- The answer
1. Use of 'will' and 'would' to express belief
(Credit for this section should go to @snailboat)
To illustrate the difference in use of 'will' and 'would' for expressing certainty, snailboat suggested the following sentences at the ELL chat room:
1A. I'm sure they have been looking for those bank robbers.
1B. I'm sure they will have been looking for those bank robbers.
2A. I'm sure they had been looking for those bank robbers.
2B. I'm sure they would have been looking for those bank robbers.
In sentences 1A (they have been) and 2A (they had been) the speaker knows they have/had been looking for those bank robbers. The main difference is that the use of present perfect in 1A (they have been) indicates that the search has lasted at least until now.
In sentences 1B (they will have been) and 2B (they would have been) the speaker expresses a belief:
in sentence 1B (they will have been), the speaker expresses the belief that they have been looking for those bank robbers.
in sentence 2B (they would have been) the speaker expresses the belief that they had been looking for those bank robbers.
2. Other uses
Again, a word of caution, the interpretation above is not unique and can change if the context changes. This section shows other possible uses of will have been and would have been.
3. The answer
Let's now consider the example in your question:
A： We saw a police helicopter yesterday morning.
B： I'm sure they would have been looking for those bank robbers.
To be able to use "will have been" is necessary to change the context as described in the first section of this answer:
A： We have seen a police helicopter.
B： I'm sure they will have been looking for those bank robbers.
Your sentence is just fine grammatically, but whether 'passionate' is the ideal word to use depends on the precise meaning you wish to communicate to your readers.
Using 'Passionate about', 'enthralled by', 'fascinated by', 'obsessed with', or 'mesmerized by' all show strong emotional feelings about the subject.
Using 'interested in', 'attracted to', 'absorbed by', 'immersed in', and 'preoccupied with' all show a strong liking for the subject but the emotion implied is less. Possibly because you have a more intellectual feeling rather than an emotional interest in the subject.
Best Answer
All the examples provided are proper uses of the phrase 'most unfortunately'.
The example in question could easily have been written as: Most regrettably, I haven't found anything. As you guessed, the introductory phrase is adding empathy and sorrow.
For the examples 'the most unfortunately named places' and 'the most unfortunately placed ads' the phrase 'most unfortunately' is adding a sense of regret and unsuitability.