1. With an RMS trigger no. It is defined as a prerequisite for the relevant age upgrade. If you want to get around this you would need to find a RMS command that grants a technology (e.g. make Age 2 xxx Active). This would have the effect you are looking for.
2. The spawned myth units are built into the techtree(x) logic and are autogenerated, eg. Age 2 Athena includes:
<effect type="CreateUnit" unit="Minotaur" delay="2000.00" generator="Temple">
<pattern type="Leaving" speed="0.00" radius="0.00" quantity="1.00" minradius="0.00">
<offset x="0.00" y="0.00" z="0.00"></offset>
</pattern>
</effect>Other technologies fire off a respawn tech that is volatile and recreates a unit if their number drops below a certain level, eg.
<tech name="Pharaoh Respawn" type="Normal">
<dbid>419</dbid>
<displaynameid>11185</displaynameid>
<researchpoints>0.0000</researchpoints>
<status>PERSISTENT</status>
<delay>90.0000</delay>
<flag>Volatile</flag>
<flag>HideFromDetailHelp</flag>
<prereqs>
<culture>
<culturename>Egyptian</culturename>
</culture>
<typecount unit="Pharaoh" count="1.00" state="noneState aliveState " operator="lt"></typecount>
<typecount unit="SettlementsThatTrainVillagers" count="1.00" state="noneState aliveState " operator="gte"></typecount>
<typecount unit="Pharaoh of Osiris" count="1.00" state="noneState aliveState " operator="lt"></typecount>
</prereqs>
<effects>
<effect type="CreateUnit" unit="Pharaoh" generator="SettlementsThatTrainVillagers">
<pattern type="Leaving" speed="0.00" radius="10.00" quantity="1.00" minradius="0.00">
<offset x="-8.00" y="0.00" z="0.00"></offset>
</pattern>
</effect>
<effect type="TextOutput">11567</effect>
</effects>
</tech>If you want to stop this happening you would need to modify the initial unitcreate tech (probably not what you want for RMS) or accept that one unit would be created but inhibit the respawn tech by making it unobtainable (if that is possibly in RMS).
3. Again in the techtree(x) this is fairly straight forward, just set enable to -1 for the specific proto unit. In pure RMS I am not sure how you would do this, unless there is a RMS command that allows you to change the maximum number of a particular proto unit (build limit).