The hasl joint looks fuller (and is easier to inspect)because it has more solder volume.
Think about it. Your pre-plating your board with solder (before you add more solder). I doubt your getting 0.3 mils of hasl.....probably more like 1.3 to 3.0 mils of hasl. This increases the volume at the lead (which is typically the hottest part of the joint). Hence the joint looks fuller. The gold may produce a frosty appearance to the joint depending on your gold thickness (5-30 micro inch?). Gold is instantly disolved into the solder and may change the appearance features of the joint. Excessive gold scavenging will cause joint embrittlement. But this is typically in joints that see over 5% (wt) gold.
The thermal profile is always important. I can not stress this enough. however, as a rule of thumb hotter (relatively speaking)tends to be better when soldering to nickel. Avoid the low side of your process temp. window. Also, aggressive fluxes will assist the cleaning and promote wetting on Ni. However, if you have good wetting spread on the gold you're probably ok (temp wise). If you have a single homogenous solder mass, you're probably ok. But, again, the profile can not be rulled out.
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