If your theory is right, then the reason the og loneal get involved with the group or trying to stop Alec is because of lily. Since lily is his best friend that dear to him, maybe he's afraid that lily also gonna die because of Alec like the first holy bride..
What I don't understand, how could Alec come to love loneal (or more interested in him)instead lily..
We know that love potion is fake..
Typical reincarnation gone wrong maybe, like in the story the holy is supposed to be Lilly but the reincarnated soul instead of going to lily went to Lineal, hence why he had the power, cuz our oh Mc is the holy bride reincarnation that might have reincarnated from our world and saw the story/game of their own world and when they died they went back and reincarnated to that world -this world. I might be wrong but it could be possible since this kind of story setting has been used before (or. Who mad me a princess and Raeliana)
Typical reincarnation gone wrong maybe, like in the story the holy is supposed to be Lilly but the reincarnated soul instead of going to lily went to Loneal, hence why he had the power, cuz our og Mc is the holy bride reincarnation that might have reincarnated from our world and saw the story/game of their own world and when they died they went back and reincarnated to that world -this world. I might be wrong but it could be possible since this kind of story setting has been used before (or. Who mad me a princess and Raeliana)
Sorry for my long ass reply...
So based on what i read in previous chapters, there seems to be at least one fond memory of Alec's where og Loneal gave Alec a potion and helped him out. That may have been the catalyst of Alec's feelings, or maybe one of several fond memories that he had of Loneal that we just haven't seen. But based on that, it's entirely possible that Alec may have either had a crush on og Loneal from afar (they are in the same classes at school) or they may have even known each other as friends.
Alec most likely has been gay the whole time, and because Loneal only had the limited scope of the novels to run off of, presumed that he was straight since in the books he loved Lily. But as we've seen, the novels didn't tell the whole story, and there's lots of details from the novels that no longer match the novels that Loneal knew (like who is the Holy Bride) or weren't even in the novels, and Alec's true sexuality might be one of those major differences. Because Alec is royalty, and therefore expected to marry a female and have royal babies to continue the royal line, he probably felt he had to hide his sexual orientation to meet everyone's expectations and buried his love for Loneal.
Now other people have noted that it's highly likely that the love potion only amplified feelings that were already there. So if Alec still had feelings for Loneal, even after his brief stint with the robed asshats, the potion would have brought those feelings out.
We're not sure how long before current Loneal came into the picture that the og Loneal was involved with the dark robe people. It may have been a recent thing prior to the explosion, which also might be why current Loneal didn't see it in the novels from his world (because Loneal was a minor character, so recent character developments for him in the first book probably got buried). If og Loneal joining the robed assholes and starting to harass Alec was a recent thing, it may be possible that Alec was on edge in the beginning because he couldn't figure out why Loneal had changed so much in such a short time (friend to asshole to cute baka). Just because og Loneal might have been a dick to Alec for a short time doesn't erase his feelings for him, it just complicates them and would probably make Alec more cautious and observant around Loneal to figure out what's up and if this person he loves secretly is in trouble or something.
To add on to your theory, the og Loneal might have thought that Lily was the holy bride (whether the group told him that or he made his own conclusion idk) and he felt it was his duty to protect her. But as time goes on he might have realized his feelings/attraction to Alec and could feel that Alec felt the same. I think that og Loneal might have figured out that he is either a descendant of the holy bride (a family secret and might not needed to be known since there are no daughters) or that he was chosen by the gods to be the holy bride (feelings towards the evil prince might be used as a attraction to them because of being the holy bride and the pull for the catalyst to happen for a repeat of the purification of the prince which can still be true if og Loneal is a descendant of the holy bride). Once realizing this he ends his association to the cloaked group so then they tried to get rid of him. Which leads us to now...
Which lead us to now..
So isn't that the start of the story, cuz didn't Loneal died or something, like cuz he's supposed to assault Lilly in her sleep and the prince saw this and all hell broke lose, and instead of getting punished by "assaulting" lily which didn't happen, it still looks like it happened cuz plot, but since our Mc is Loneal now he kinda changed it and didn't get punished but instead got the prince fall in love with him...
Which would lead us to a lot of misunderstandings and typical cast not communicating...?( ̄∇ ̄")
Okay, so we have 2 mysteries as of Chapter 49: Alec's mysterious "bloodlust" dark aura/voice in his head thing, and Loneal's weird painful cramps back in the Water Temple that the bard dude healed.
Looking back at Chapter 20, where they're in the library, Alec tells the legend of the Holy Bride (what Loneal most likely is). In the story, one of the royal family falls into darkness and gains like dark magic/wings/etc. and tries to destroy the kingdom. The first Holy Bride (whose is supposed to be Alec's ancestor, along with the god she's married to), dies while stopping the evil royal by dispelling his evil magic. However, it's unclear if the evil was wiped away or absorbed by the Holy Bride.
So what I think is going on is that when Alec got his "bloodlust" (aka the evil like that royal from the legend, when his eyes go all dark and his flames take out a shadowy/inky look) back on the cliff when he was fighting Mr Robed Asshole, Loneal managed to dispel the bloodlust with his Holy Bride powers. But dispelling the bloodlust may not mean getting rid of it, but instead may be that Loneal is absorbing the evil into him, which caused the painful spasms after he fell from the cliff. I think those spasms were the evil aura eating away at his insides. Because that would explain why the first Holy Bride died.
Another issue this theory brings up: it may be possible that Alec might not be a descendant of the first Holy Bride, but instead the evil royal whose evil aura killed her. That bloodline would explain the voice in his head and why it keeps happening.
Here's where I get all conspiracy theory crazy. That knowledge might be known only to the royal family, because who the hell wants to follow a family whose ancestors murdered the Holy Bride? It's possible that Mr Robed Guy's group found that secret out via old Loneal's research (or other means), and the reason they were harassing Alec in the beginning of the story was to try a trigger his bloodlust. Maybe they were trying to have the bloodlust exposed in front of the public to denounce the royal family, or maybe just to target Alec specifically, who knows. That may be why Robed Bro was trying to kill Alec in the forest early on (because he knew Alec's bloodlust was a threat and wanted to stop him before it activated), and why he was trying to get Loneal during the cliff fight (trying to stop him from dispelling the bloodlust with his magic, which would ruin any plan to kill or expose Alec).
Then there's why the original Loneal would be involved with this group trying to stop/get rid of Alec. That I have no clue about that. Okay, essay over. Thank you for attending my Ted Talk.