Well i had a similar discussion years ago - i want to spare us the long story - the result:
BattleTech already has a energy system - called heat.
With a proper system you can manage your ideas with heat.
Although i really (insert more really) like the idea of the PEC. And of course the energy drain/ heat gain for reloading.
Both systems could really help to increase the immersion:
i have made my on thoughts - based on your ideas (including reload and PEC)
I take the BJ-2 as an example:
Whats new?
The weapon system: the ERLarge Laser - produces much more heat (22) while firing and additional 10 while reloading (rough multiplication of 3-4)
Even the cool running SSRM2s - produces more heat as currently.
Because of its 11 DHS (old idea) it got a threshold of 11 before the real "heatscale" starts.
The main difference the dissipation is multiplied by 4
now about your ideas: engine is a 180STD - it produces 1.8 heat per second - when running 100% (lower heat when walking) - each maneuver costs heat too.
Even the targeting - need energy - and produces heat - those "standard" tasks have Priority 3 (P3) - while reloading has for example (P1). When heat rises the heat scale may reduce the energy for moving or targeting - when increasing the priority of those tasks you can overcome those shortage - but it will cost more heat (reduce the available dissipation)
Now a example for Alpha or Chain Fire (not even using the ER Large Laser)
a really raw sketch for a raw idea:
Firing all 4 SSRMs will heat up the Mech quite a bit - with a P1 reloading the dissipation is reduced for additional heat point per second. The pilot can give the reload mechanism more time - and reduce priority to P2 (8sec) or P3 (16sec) - and reduce the heat burden drastically
those heatscale penalty's would reduce the ability of the BJ 2 to move or to target enemys - of course the pilot can give different priority - he can even over tact his reactor creating a negative dissipation - the mech will over heat and destruction or shutdown is inevitable.
as an option there is chain fire:
The warrior can fire up to 6 or 7 volleys (did not bother to calculate) without penalty's the heat burden is increased slowly while firing all missiles in chain - rather than together. So the waste heat can be removed faster.
As you see - i have taken the very good ideas of the OP and added my own findings. It is a more complex system of course - to prioritize different systems need more micro management - but hey most of MWO players are old - most of them already know this stuff from games like WingCommander; X-Wing; TIE-Fighter (to change the Energy level for 4 systems shouldn't be that hard)
Hell i really really like the priority idea. It really helps to overcome my problem when "inventing" a fair system (heat spike but much more dissipation) because my ideas did kill the light harasser - that really need to Alpha Strike a target - now they can give reload and targeting a lower priority and increase the priority of the movement. So they can make
one slasher run without loosing speed or being forced to chain fire - but can't do a circle of death either.