Ok, first things first: Havocs.
Like last year and many years before, there is the “count-as” rule in place. This is a rule to allow people some flexibility (within reason) to align their miniature collection to the rules of the
ETL. A melta gun may count-as a plasma gun if needed and a power axe as a power sword. Obviously this should be used with fair play in mind .i.e. you may not have a Marine to count as a Dreadnought. In short you may not gain a points’ advantage via that rule and you should expect the effort-to-points ratio to be equal or worse than when pledging the original item.
In you case I’d say that the rotor cannon is an excellent “count-as” for the Reaper Chaincannon that is a legit choice for the CSM Havoc Squad. So no problem there.
Where I can find an issue however is squad consistency. Havocs in the latest Codex are strictly a 5-strong unit with no option to increase it. Furthermore, the Aspiring Champion squad leader does not have the option to carry a heavy weapon himself. So you need to reconcile those facts...
Alternatively, you may vow them in the
HH forums as a Tactical Support Squad armed with Rotor Cannons. You have no problem with model count there as the minimum squad size is 5 and can have up to 5 additional Marines while the squad sergeant may be identically equipped.
These are your options. Obviously if you want to vow other Chaos Marine stuff then that means you may not pledge any units for
HH unless as out of competition entries. In than case you may still paint them, show progress in the
ETL threads and generally use the
ETL as motivation, but no points will be awarded to your Forum for this particular effort (I.e. if you just paint minis that do not constitute a game-legal unit).
On the bases front, well, there is no set rule. The mini is done when you say it’s done. However, it is expected that you play fair and paint to your usual standard - if not improve on your usual standard. As such, if you normally base your minis, it is expected that you do. There is no hard rule so as to cover the possibility that people who do not posses the skill to do elaborate basing do not feel excluded or discouraged to take part. On the contrary. But if you do posses the skill, if you do base your minis, you’re expected to do so in the
ETL too.
The
ETL always promotes quality over quantity. It should not be viewed as an opportunity to cut corners, rather as an opportunity to paint to the best of your ability or even to improve your skill.

Phew... That was a long answer...