While I'm not sure about your theory on the drop rate being relative to the amount of people farming him, I totally believe how many matches you have played. If I were to err on the minimal side, I would offer that I have played the Imperial Indolent at least 3000 times. The matches are fast and the MGP is good.
Ironically enough, I have gotten some of my best cards while rushing through a match because my duty popped(there is my anecdotal theory)
But literally: Bahamut, Odin (from Landenel), Kan-E-Senna and 2nd Merlwyb drop all came with less than 15 seconds on my duty finder whilst rushing a win lol.
It will be very beneficial to your ability to amass MGP quicker and more efficiently to definitely just take that time to get to 60 cards. Indolent's deck has absolutely nothing against a Scion deck in Ascension. I can literally afk-watch Netflix if I wanted and win against him with the deck. But I don't do that, because I love playing Triple Triad.
It's also why I don't really mind not getting Gaius as well. I am going to play that NPC to farm MGP anyway. I'll get it eventually. I'm not sure where you are in terms of your card collection, but I would definitely take the time to get the 1* cards from the dungeons. The drop rates seemed pretty generous imo. The only dungeon I needed to do more than twice was Pharos Sirius, and I believe that is based on Siren being a 3* card in a relatively low level dungeon in terms of the 50 dungeons.
It was also my experience that the 1* cards you could get from either HM or NM dungeons, had a better drop rate in the NM dungeon. This has had different reports from different people though, so don't take my advice at face value. All I can say is that I did each HM version once first, and for all of them I got the drop on the 1st dungeon I did in the NM difficulty, except Tonberry took me 2 WP NM''s to get.
I have a pretty good descension deck, so getting wins against Hab doesn't really deter me that much. I really want Fenrir ^^ so I'm going for grind sessions against the Indolent until I get Gaius and it forces me to move on lol. But yeah, I think I dread Rowena/Swift the most. Even with a 3* deck, Chaos still makes consistency against Swift tough. I can beat him much easier now with a 3* deck, but the time window coupled with Chaos makes matches tougher to come by, let alone wins.
And Random is, well Random haha...at least Louisoix is only a 4* card. Drop rate should be a bit more favorable than the 5* drops from NPCs