Need Help! Pocket in stainless

The part is a handgun slide with a surface-hardened Melonite coating. I am machining a 1/4" pocket (with 1/16" radius corners) and was hoping to run a pocket with the 1/8" tool. I'm manually adding Tapmatic while trying my best to keep the chips clear. I first tried an 1/8" 3 flute carbide at 768 rpm and 1.4 feed (0.000625 IPT), .48 plunge and it snapped on the plunge. After exhausting my only carbide tool I switched to a HSS 4 flute and a toolpath that ramps to depth using same specs but a 1.9 IPM feed (using the same 0.48 IPM feed during the ramp). It trashed the face of the end mill after the first 1.5" of cut.

So...where did I go wrong?
1) Is my feed/speed calculator off? I usually drop sfm/rpm by 60% because I'm manually adding oil, should I be doing the same on something moving this slow (and has such a high risk for work-hardening)?
2) I realized that I ran the carbide at HSS feed/speed. Is that a problem? Does it NEED to be ran like a carbide?
3) I fear that I'm getting some work hardening. Should I be using plunge rate or feed rate during the ramp?
4) What kind of chipload should I be seeing?

I have already done some machining on this slide, but I had the luxury of endmilling in at depth and not plunging. This cut is obligated to plunge.

So my work-around was to buy a 15/64" end mill to plunge as beefy of an endmill as I can in to rough it, then finish the cut with the 1/8" so it enters the cut at depth. It will be here tomorrow.

What do y'all think?