1) No problem and I get it. I was just pointing out that Infinity isn't like 40k in that--without knowing what the list you're playing is--I can't rule out the vast majority of equipment/special rules from being in the army as they're aren't faction locked in the same way. But yeah, figuring out what your army is and then practicing the basics of order of operations and what specific equipment does in your list is a good way to get started in most any game and especially in Infinity.
2) There's a button for list validation in the official army builder and, for the most part, it's correct. I haven't seen the validation ever be wrong, but I have seen the stats be off once or twice before getting corrected so that's something to be aware of (although the general way of figuring out what version is correct in Infinity is FAQ>wiki>army builder>army book).
Duo is especially confusing because it's one of the Fireteams that non-sectorial lists have access to, but only if they bring an EVO REM (remote) and spend one of their command points at the beginning of the game. It's super niche, but it's there.
3) That was someone either trying to be funny or trolling. There are no sectorials or armies that are on the army builder that are not allowed. There are formats that change what models are legal where, but that's a whole different thing and even then the sectorials on the army builder don't just go away. Corregidor is fully legal.
4) Pilots are a little weird as the models were added a fair bit after they started producing TAG models. Technically you need to purchase a separate model for pilots (which the Gecko has an official one if you'd like), but you only need one if you plan to get out of your TAG/if your opponent forces you out via hacking. Both of these are pretty niche practices as pilots leaving their TAGs make both models super vulnerable and disembarking tends to be a more advanced form of play. As for the hacking example, it's one example and widely considered a waste of resources in most circumstances--especially against small potatoes like a Gecko.
So, yeah, you technically should have a model that you can produce for the pilot of your TAG, but it won't be used very often. (Although most people collect the pilot models as they're some of the more interesting Infinity models and make for pretty cool HVTs when you're not bringing the TAG).
5) Infinity has an...interesting modeling policy. The game is both one where proper representation of your models is important for the clarity of the game state and one where to make every variation of unit type would be too limiting a factor for retailers and collectors so instead you can sorta use anything as anything. Granted, this is technically anything reasonable, but given how subjective that statement is it can get a little weird.
For the most part, people will expect you to use the model that matches the profile you're fielding if it exists (so WYSIWYG) and, if it doesn't, the best practice is to use the same type of model as what you want with a proper weapon swap to the profile you're fielding (e.g. chopping the Combi-rifle off of a Hellcat and replacing it with a Spitfire to represent a Hellcat with a Spitfire). When no model exists, people tend to either convert their own version or use the Spec-Ops models to represent them (Spec-Ops are any of the models in boxes marked Dire Foes or marked Spec-Ops).
That said, you don't need a special model to represent skill sets--just equipment. In your Lt. example, this would actually be frowned upon as who the Lt. is in a force is hidden information in almost all Infinity missions. You do not declare and should not visibly mark who your lieutenant is on the model as it removes some of the gameplay along with putting you at a sometimes-massive disadvantage.