I think the text is somewhat vague on the question, but, to me, anyway, it's still very obvious that Garak and Parmak are in "a relationship." I readily grant that this is a subjective conclusion, but it is not one I arrive at arbitrarily.
The banter of their conversation throughout the book, but particularly during their dinner with Picard and Crusher; Garak's describing Parmak as his conscience; their games of kotra with kanar before bed; the very strong implication that they live together; the degree to which they are both clearly overcome with emotion upon their reunion after Garak's "death"; the intimacy of their conversation at the very end of the book, with Parmak preemptively forgiving Garak.
They clearly have a very close relationship. And Garak is explicitly stated to "love" Parmak.
So, are they "in a relationship"? Are they "involved"?
To answer that, we need to examine the ways that we normally conclude that fictional characters are in a relationship. I will address each one in the context of this story.
1) We are told they are married. As far as I can tell, we have very little evidence about how Cardassians consummate relationships, especially outside of the old aristocracy. Certainly in post-war Cardassia, with all its societal upheaval, it would not be surprising if an old institution like marriage were on its way out. So this approach may not have been an option for McCormack, given in-universe constraints.
2) We are told they are having sex. Garak and Parmak are old, at least in the sense that they aren't young. From what I can tell, sex is an important part of relationships between older people, but it's often not the most important thing, the way it can be for young couples. Sex may not be the most meaningful way Garak and Parmak have to be intimate (assuming they are a couple); that's certainly consistent with the characters. Furthermore, discussing Garak's sex life would, I think, have felt out of style for the novel that McCormack was crafting. So this approach was also not an option for McCormack.
3) They refer to each other as lovers. Garak would never do this so bluntly, so, again, another option removed for McCormack. (Although, I think it is notable that Garak refers to Parmak as his dear friend [or whatever it is he precisely says]– that may be as explicit as Garak could believably get.)
4) Others refer to them as lovers. This would never happen in this story. Any character doing so would be gossiping in a way that none of those people in a position to know would. Furthermore, because of the constraints of the story and its characters, only a few people would know about Garak and Parmak anyway.
5) The characters in question are shown behaving as a couple does, doing the things that couples are recognizable for doing. In my opinion, this is what McCormack does with Garak and Parmak, in the examples I noted above and in the quotes which Jarvisimo provided.
Then there is the whole issue of what it actually means to be involved with someone, both in the real world and in a fictional narrative. Do you have to be having sex? Do you have to be married? Do you have to be in love?
These are very big questions. For my money, I think what we see, what we are explicitly told, of Garak and Parmak's relationship is consistent with a description of two people who are involved. But that's, in part, due to my personal conception of what such a couple looks like, how they behave, so it can be hard to justify, without writing a dissertation, anyway.
For various reasons, I think McCormack would have had a hard time stating more explicitly that Garak and Parmak were in a relationship in a way that didn't inappropriately break the fourth wall. However, I also think that she provides plenty of evidence to make it clear that Garak and Parmak are, in fact, "in a relationship."
The banter of their conversation throughout the book, but particularly during their dinner with Picard and Crusher; Garak's describing Parmak as his conscience; their games of kotra with kanar before bed; the very strong implication that they live together; the degree to which they are both clearly overcome with emotion upon their reunion after Garak's "death"; the intimacy of their conversation at the very end of the book, with Parmak preemptively forgiving Garak.
They clearly have a very close relationship. And Garak is explicitly stated to "love" Parmak.
So, are they "in a relationship"? Are they "involved"?
To answer that, we need to examine the ways that we normally conclude that fictional characters are in a relationship. I will address each one in the context of this story.
1) We are told they are married. As far as I can tell, we have very little evidence about how Cardassians consummate relationships, especially outside of the old aristocracy. Certainly in post-war Cardassia, with all its societal upheaval, it would not be surprising if an old institution like marriage were on its way out. So this approach may not have been an option for McCormack, given in-universe constraints.
2) We are told they are having sex. Garak and Parmak are old, at least in the sense that they aren't young. From what I can tell, sex is an important part of relationships between older people, but it's often not the most important thing, the way it can be for young couples. Sex may not be the most meaningful way Garak and Parmak have to be intimate (assuming they are a couple); that's certainly consistent with the characters. Furthermore, discussing Garak's sex life would, I think, have felt out of style for the novel that McCormack was crafting. So this approach was also not an option for McCormack.
3) They refer to each other as lovers. Garak would never do this so bluntly, so, again, another option removed for McCormack. (Although, I think it is notable that Garak refers to Parmak as his dear friend [or whatever it is he precisely says]– that may be as explicit as Garak could believably get.)
4) Others refer to them as lovers. This would never happen in this story. Any character doing so would be gossiping in a way that none of those people in a position to know would. Furthermore, because of the constraints of the story and its characters, only a few people would know about Garak and Parmak anyway.
5) The characters in question are shown behaving as a couple does, doing the things that couples are recognizable for doing. In my opinion, this is what McCormack does with Garak and Parmak, in the examples I noted above and in the quotes which Jarvisimo provided.
Then there is the whole issue of what it actually means to be involved with someone, both in the real world and in a fictional narrative. Do you have to be having sex? Do you have to be married? Do you have to be in love?
These are very big questions. For my money, I think what we see, what we are explicitly told, of Garak and Parmak's relationship is consistent with a description of two people who are involved. But that's, in part, due to my personal conception of what such a couple looks like, how they behave, so it can be hard to justify, without writing a dissertation, anyway.
For various reasons, I think McCormack would have had a hard time stating more explicitly that Garak and Parmak were in a relationship in a way that didn't inappropriately break the fourth wall. However, I also think that she provides plenty of evidence to make it clear that Garak and Parmak are, in fact, "in a relationship."
This book was an excellent scifi/ mystery novel.
I was unaware that Nan was having a secret liaison. Was it with Akaar? Or maybe that's who Garak was in love with... 
