For situational trash mob clearance, Rosmarinus is better against Kin and FlameSprayer agains beasts, so "generally" speaking Rosmarinus is better below ground ( Chalice dungeons ) and Flamesprayer above ground (main game).
No, it's shit for a pure arcane build. What you have now is not a converted weapon. Regardless of it being Normal/Lost/Uncanny, the Blade of Mercy can't be converted, cause it's at split damage weapon and it's arcane scaling is subpar. Here is the mechanism behind it:
This is the damage stat for the Blade of Mercy, it physically scales with STR/SKL.
View attachment 362079
Notice it's already imbued with 30 ARC , so it scales with that too. When you aim for a pure arcane build of 99 you don't want to weaken you weapon with split scaling from stats that you only meet the wield req for. So what will happen when you put your fire gem in it is this...NOTHING, everything remains the same in regards to stat scaling.
A conversion weapon (pure physical) however is a completely different story:
Here is the Saw Spear damage stat, it also physically scales with STR/SKL
View attachment 362081
Notice that there are no imbued stats for Blood/Arc/Fire/Bolt. This is what you want, and what will happen when you put an elemental gem in it is that it stop all scaling from the physical stats and put all scaling into Arcane. Now you have an true arcane weapon.
The reason you feel that the Blade of mercy "wreck things" is not really an elemental thing and maybe in parts more of a mental thing, and/or progression dependent.
You started out really rough and your point of reference to compare with is your prior build and your favourite weapon Blade of Mercy. So in the end you went back to what you are used to with maybe a little arcane sprinkled on top.
Of course it will feel much better with a lvl 6+ gem slotted split weapon compared to a low lvl conversion weapon with no gems in it, and on top of that a moveset you are not used to or maybe even like.
That is why I always try to recommend weapons that have similar movesets to "favourite" weapons but that are pure physical, for example you mentioned when starting your run that you planned on using the MLGS, (split damage weapon). I reccomended LHS instead (pure physical) , as both weapons share similar movesets.
If you are still not convinced that Blade of Mercy is bad for a 99 ARC build let me give you some stats. Let's compare Blade of Mercy with one of the weakest conversion weapons for PVE , Beasthunter Saif.
So how does BOM compare to the weak BHS in a pure arcane 99 build?
View attachment 362083
Out of 486 possible gem combinations, three different combinations were choosen to compete with one of the weakest conversion weapons, and even then the BOM underperforms in all of the test scores.
Im not suprised, atm you are just putting points into Arcane without even being close to using it to it's full potential. If you still want to soldier on with the Blade of Mercy, you need at least get to ARC 50 and start collecting your hunter tools ASAP.