New player questions.

[Mantriel]

Q1- yes, but look, if India or Canada become a major after UK fall. It could happen so late in the game.
Q2- Iran and Afganistan sure, why not (but if You want to restore Roman Empire, I don't know, if You need Iran at least for Your own). Turkey is very annoying pupet, because their focus tree is broken and they can break free and create their own faction.
Q3 - English Channel is very bad place for naval invasion on British Isles. Most of the Royal Navy is swimming here and british AI protects southern ports very heavely. Look on YT for Wilhemshaven trick - naval invasion from Wilhemshaven to Hull works for me almost every time.
For USA - it will be hard. Build 3 levels of collaboration goverments, so You don't need rush to Los Angeles. And cross the fingers, maybe there will be not that many american soldiers at home, if they fight with Japan still, but in 1943 USA are really powerful

I managed to Sealion quite easily in the channel.
I was convoy raiding from Gibraltar for 2+ years the whole time and I was researching naval stuff and building ships the whole time, so apperently I had a greater navy than the Brits + Vichy + Germany + a massive Air superiority).
Took out the UK in 40 days, Japan flipped sadly and became a major on the Allied side. Only USA and Japan left. The Raj is a minor.

I don't plan to do the Roman Empire achievement right now. I am happy with just a single win (I don't have any yet).
I don't have any DLCs (well actually I am going to buy them soon, but I don't intend to use them yet).

Q.: Marines.
Q. 01.: Is there a point to marines at all? My normal troops can marine invade just fine, I see no difference. Is there a reason to do the research and spend the army XP?
Q. 02.: Sometimes my naval invasions take 20-ish days sometimes 40-ish days... I guess this has something to do with the number of sea tiles? Are marines faster? I had the impression that they are not.
- - -
Q. 03.: After getting my hands on the British industry I was wondering what should I build with them against the Yankees? I have a pretty basic infantry army right now (with some artillery) and 4 divisions of light tanks (and a huge Air force + huge naval force). I was thinking heavy tanks (although I would have to research it and build it, so probably not the best idea).
Q.04.: I tried out the various support companies and I had the impression that signal company don't do anything for me. Honestly Field hospitals seem to be the best for my playstyle (no DLCs, a lot a manual micromanaging). My four favorite are

1. Engineers,
2. Logistics
3. Anti Air
4. Artillery
5. Field Hospital

I didn't try flame throwers or armored recon yet.

Q. 05: If I have an "army" of 4-6 divisions of light tanks (20-30 width, 40-60% or 50-50% mechanized + light tank) is there a reason to mix up the support companies in the individual divisions?
For example make them 30 width use groups of 3 divisions, one of them has recon, the other two signal or something like that?

 

[DystopianAlphaOmega]

Q. 02.: Sometimes my naval invasions take 20-ish days sometimes 40-ish days... I guess this has something to do with the number of sea tiles? Are marines faster? I had the impression that they are not.

Two things - number of divisions per invasion and naval invasion tech. Sending two divisions on the same naval invasion takes twice as many days to prepare compared to sending each of them separately. The number of days to prepare per division (regardless of division type) is determined by naval invasion tech. The 1940 upgrade is a pretty noticeable improvement.

Marines are helpful against opposed landings and things like river crossings. If the defence is poor enough to land around the port and take it before you run out of supply, they aren’t as useful. Special forces do also get some other bonuses from things like the special forces specialization techs and iirc have slightly better base stats than basic infantry for the most part.