Just to be clear on how Diplomatic leaders from the core optional rules works, since it is confusing and the FAQ helps only a little.

If I try to invade an enemy planet with diplomat, my opponent can use the diplomat's ability to stop the invasion. According to the FAQ it is only for that invasion attempt and the diplomat cannot use that ability again for the rest of the round or the next game round. (a game round being from Strategy to Status phase.) This means that another player can invade or if I have the warfare strategy card then I can activate the system and invade again without diplomatic shenanigans. Just want to make sure I am tracking this correctly.

Now the value of the diplomat does not seem great but if used at the right place and time could buy your opponent time. Even if you do happen to have the warfare card he could send a fleet into the system, build more troops/PDS on the planet (if he has a space dock), or even use the diplomacy strategy card to prevent an invasion that round.

I can definitely see needing fore thought when attacking a planet with a diplomat. Make sure you have the warfare card and he does not have diplomacy. It could also be a problem if your opponent has allies that snag the warfare card and your opponent gets the jump on you in selecting diplomacy.

Of course, that does not help his other planets and systems from attack but the protected planet may be strategically placed. Taking out this planet as your opening move while he does not have diplomacy and you have warfare could be key.

What happens when there are 2 diplomats? I am not sure if any of the races have more than one but could 2 keep a planet safe over multiple invasions.

Of course many other actions cards (like signal jamming) could make taking the planet a real headache and your target opponent does not even need to be the player to play it. Even if not his ally they could play the card to slow you down.

Basically diplomat leaders have can potentially be more powerful than they appear.

If I try to invade an enemy planet with diplomat, my opponent can use the diplomat's ability to stop the invasion. According to the FAQ it is only for that invasion attempt and the diplomat cannot use that ability again for the rest of the round or the next game round. (a game round being from Strategy to Status phase.) This means that another player can invade or if I have the warfare strategy card then I can activate the system and invade again without diplomatic shenanigans. Just want to make sure I am tracking this correctly.

That is correct, according to my understanding.

What happens when there are 2 diplomats? I am not sure if any of the races have more than one but could 2 keep a planet safe over multiple invasions.

There are definitely races that have more than one diplomat. Xxcha, I'm pretty sure, has two. As for what happens, the answer is pretty obvious. Having two diplomats on a planet gives them two opportunities to veto an invasion of that planet per round. Or they can be used on alternating rounds to keep the planet protected all the time (once per round.)

Basically diplomat leaders have can potentially be more powerful than they appear.

Diplomats, like so many other things in this game, are situationally powerful. In the right place and at the right time, they can be awesome. However there are ways around them, as you have already suggested. Also, they don't help your opponent much if you have a strategy that doesn't involve invading the planet they're on.

What happens when there are 2 diplomats? I am not sure if any of the races have more than one but could 2 keep a planet safe over multiple invasions.

There are definitely races that have more than one diplomat. Xxcha, I'm pretty sure, has two. As for what happens, the answer is pretty obvious. Having two diplomats on a planet gives them two opportunities to veto an invasion of that planet per round. Or they can be used on alternating rounds to keep the planet protected all the time (once per round.)

I don't think this is correct. The rule for diplomat states: "A planet protected by a Diplomat may not be protected again by a Diplomat for the remainder of the round, or for the next game round."

Thus the diplomat can use his ability any number of times in the same or next round, just has to do it in different planets. In the other hand, the protected planet cannot be protected again this or next round regardless the number of diplomats present on it.

So it looks senseless to have 2 diplomats on one single planet that is subject to invasion after being protected once.

Oh, you're right.

I forgot, the only leader whose ability stacks is the General, per the FAQ.

Just to be clear: the planet loses its ability to be protected; the diplomat does not lose his power to protect.

If I have a diplomat on Quann. I prevent an invasion in round 2. I somehow move him to Lodor. Now he protects Lodor, (quite possibly during round 2 also).