Well of course she enjoyed the battle. She's a Kenpachi, that stuff is literally in their blood. We saw Kenpachi smiling like a psycho despite being nearly bifurcated by Gerard's blade. Their actual state/condition hardly impairs their enjoyment.
Does it matter that it takes from both? Point is, in a proper fight, she would be using that to heal herself instead. That was actually why she learnt healing arts to begin with. It was to prolong her fights.
How can Zaraki be the one with a handicap? He was literally getting stronger each time (well more unsealing his powers but same thing). It doesn't matter how easy Unohana had it at the beginning. It would steadily gotten harder each time she killed him until the time he eventually did kill her. It's more reiatsu for zaraki with time, and less for unohana as well as her healing his wounds with each passing. Yes, towards the end, they went at it with full strength but that doesn't mean they both automatically were restored to their peak, well, except Zaraki.
True, KOing multiple SRs might not be a clear decisive thing anymore but it isn't exactly irrelevant. Also, it's not really that Byakuya and Hitsugaya have been helpless or useless. The thing is that they're not a quarter as reckless as Zaraki. Zaraki came here to have a great time and damn the consequences. Byakuya and Hitsugaya are considering possible repercussions. We also saw that despite Gerard's numerous attacks on Byakuya, the latter is relatively unscathed. Also, Byakuya did block Gerard's attack aimed at Hitsugaya iirc. It's not that Byakuya hasn't been helpless, he knows that cutting up Gerard isn't going to do anything but make him stronger so he wouldn't indulge in that and as mentioned. Zaraki nearly got killed due to his bullheaded attacks. Right before Yachiru showed up and offered bankai, Zaraki was pretty much done but Hitsugaya and Byakuya were still up and about. Zaraki didn't exactly fare any better against Gerard. He just opted for straight attacks regardless.
Actually, the heat itself aren't flames. That's just the form his reiatsu appears at. Further more, I checked and it's actually a technique, it's West of Zanka no Tachi. It's not the flames that induce the heat, that's the appearance of his reiatsu. The actual heat is a technique.
As seen there, haschwalth correctly states that such heat wouldn't be expressed in flames. Yama allowed that to show in order to give a visual representation. So the flames that appear is indeed his reiatsu which can indeed be resisted. As in, Zaraki wouldn't start gasping or anything. But the heat of the technique itself isn't visible but it still in effect. Fake Bach makes the point that he literally feels like he's burning. He states that it's only because of Blut Vene that he isn't dead yet but that doesn't prevent him from feeling the effects of the intense heat. It's not something that can be bulldozed by reiatsu. Also, all attacks are based on reiatsu, even up to kido. To say that they can be resisted like that literally means that Zaraki is impervious to attacks of any pretty much any nature. I'm not saying Zaraki would get obliterated by ZnT West but it's a pretty key point. He has no actual defense to shield him from the heat and its effects.