Is there an instruction manual somewhere?

No instruction manual. Learn to play through the tutorial and then learn by experience and paying attention.

Thanks for the link. The vid and some of his other vids were very useful.

Dunno why the developer keeps that stuff secret, tho. Maybe I'm old fashioned (certainly, I am!), but I do think that a developer should at least explain their interface. Trial and error is a stupid and frustrating way to learn a game. At least, for me it is that way.

The rules and systems are not hidden...they are right here:
http://www.drivethrurpg.com/product/217189/BattleTech-BattleMech-Manual?src=hottest_filtered&filters=0_2140_10132_44499_0

The Battletech Manual for the table top game. All the rules for the PC game in one convienant form. Plus being able to play the Table Top game as well! Figurines not included.

Errr, no. They made all sorts of decisions about how those rules should be implemented. And the rules certainly don't explain the meaning of all these random chevrons that litter the interface. Or how you get your guy to execute a DFA. Or how you get your guy to run around behind and melee rather than run around behind and shoot. Etc.

The chevrons are explained well. They represent your evasion pool. The further you move the more evasion you get which acts as a direct penalty to attack. This is tha same as in the TT game. Yes in the TT game it is a flat penalty to attacks on the target for the duration of the turn, not a lossy thing.

But with just a little knowledge and a touch of thought you can easilly see they stuck very duitifully to the original rules. Only changing things that do not directly fit like the amount of armor on 'mechs. Heck they are even using a hex based movement system in the game compared to a cubic one in games like X-Com, and this is also directly inline with the rules.

THE ONLY thing I have yet found that I did not understand is that TECHNICALLY armless mechs like the Jenner and your starter mech (Hat that thing) should not be able to make physical attacks. But that is a quibble and likely a design choice that had to be made to prevent people asking "So why can't I kick this thing if I can not punch it".

You did not look very hard for chevrons, I notice. Anyway, be well.

I played tabletop CBT and Megamek for the better part of a decade, I wouldn't say that they "stuck very dutifully to the original rules".

The interface is obtuse at best and terribly explained via tutorials.

on that regard, i agree with OP... The whole UI is terribly explained (or not explained at all)

i mean, i miss the good old video games manuals where you had some screenshots with numbers explaining which parts of the UI were which .
The ingame tutorials are mostly about useless and obvious things, frankly...

even if i love the game so far, it took me 3 missions at start to figure out where was the heat meter was, the whole stability concept is poorly explained (i know you fall once all 5 are filled up, but yet i dont know what is the stability threshold that can be at 2 or 3, with appropriate perk)

the evasion chevron and the evasion effect too is barely explained, and the numericla effect on shooter's aiming.
Gunnery says it grants bonus % to the "base aiming" of the weapons, but i am not sure if every weapon share same base, or if each weapon has a different base aiming rating...

i saw a streamer who discovered the game the other day, and the guy was completely clueless and got so shredded up, even if winning, throughout the first 3 missions that he was going in a dead end with all damaged mechs, no money to repair and mechwarriors injured for 2 monthes each :)

even if some stuff are different in this game than in the boardgame and tabletop RPG, and even if some concepts (like pilot perks) are new... i think could consider myself quite favored of having some long ago experience of the originakl board/rpg tabletop games, and am an overall tactical games lover (xcom, xenonauts, battle brothers, and litteraly any tactical stuff that can fall under my filthy fingers) but i cant help but thinking most genuine beginner players would be quite lost...

of course, if that does mean they would have to dumb the game down to catter the widest audience and over-holding hand like many games nowadays do, then, i'm fine with current situation. Still, i think somme middle ground and "welcoming"/proper information and rules explaining couldnt hurt the game

In tabletop, ramming is also an option and technically every mech can make physical attacks no matter how short and stubby their arms are (with the caveat that having lower arm / hand actuators increases your chance to hit).
The base rating comes from your pilot's gunnery. When in mission and targeting an enemy, hover over the weapon in the weapons list to see the bonuses and penalties that affect it.