Okay, thanks for the honest and candid reply. If you can give me a short and detail answer, that will be much appreciated.
1. costMelee and all similar attributes will remain unchanged, they work 100% as intended... any change would be a bug
2. I am not planning to make melee do AOE damage
3. I also don't see any reason for battleType=12, it's not the weapon that explodes, it's the unit that explodes/self-destructs
4. Self-destruct can be a simple false/true flag on a unit... the actual self-destruct would be a new code, not reusing any existing mechanic... it would also be completely independent from leeroyJenkins and everything else mentioned above, I don't see any reason to "bastardize" (sorry for expression) well-defined existing features
5. The parameters of the self-destruct explosion (type, power, etc.) would be defined either via a dummy item linked to the unit; or by a few new attributes on the unit itself
All new attributes are subject to change during actual implementation, depending on compatibility with other features and potential extension of cyberdisc/bio-drone self-destruct triggers, power and type.
6. terrain destruction by melee is completely offtopic for self-destructing units... and will be handled completely separately... not going to describe it here
Lastly, of the two path I laid down above, which would you go to code it. If you approve of this feature.
None. They are both bad (in context of OpenXcom).