Unleash Incarnation cannot exceed haste's limit
Haste states that the target
gains an additional action on each of its turns. That action can be used only to take the Attack (one weapon attack only), Dash, Disengage, Hide, or Use an Object action.
Haste does not set a baseline of one attack to which you can add more. Rather, haste sets a limit on the number of weapon attacks you can make with its Attack action. Hence, you cannot make both the normal attack and the Unleash Incarnation attack when you take the haste's Attack action, because that would exceed the limit of one weapon attack.
Moreover, the Unleash Incarnation attack is not some attack that occurs after the Attack action, nor is it separate from that Attack action. Rather, the Unleash Incarnation attack is part of the Attack action, so it's subject to the limits that haste imposes on the Attack action.
The above is the exact same reason why Extra Attack also can't exceed haste's limit. As a matter of fact, the relevant wording of Unleash Incarnation and Extra Attack are the same:
Whenever you take the Attack action, you can make one additional melee attack from the echo's position.
you can attack twice, instead of once, whenever you take the Attack action on your turn.
Echo Knight is sloppy and is going to require some work from your DM
At this point, we can't avoid saying it. Usually, WotC writes very tightly and with great purpose. Echo Knight, however, has a lot of problems. If you want to play this class, make sure your DM is up for making a lot of rulings.
It seems clear from the language that the intention is for your reach to encompass the space around your echo, but the language is so imprecise and vague. The rules for reach are just kind of hinted at.
RAW reading - the grappled stays grappled
I'll try make a strict raw reading of the rules for you, but like I said before, this is going to be up to your DM.
The echo can be used to grapple: from Manifest Echo: "When you take the Attack action on your turn, any attack you make with that action can originate from your space or the echo's space.", from Grapple: "When you want to grab a creature or wrestle with it, you can use the Attack action to make a special melee attack, a grapple." So for starters, you can use the echo to grapple.
The next problem is the Grappled condition, as you note: "The condition also ends if an effect removes the grappled creature from the reach of the grappler or grappling effect" notice the use of the word "removes"?
Remove means: "take (something) away or off from the position occupied." or "abolish or get rid of." The question is, has the grappled creature been removed from the reach of the grappler or grappling effect? From my reading, no. The grappled creature was never within your reach, therefore it could not be "removed" from reach. Alternately, if Manifest Echo is a vehicle for a "grappling effect", then the grappled creature has remained within reach of the grappling effect.
No. The Echo Knight is still making the attack; it's just coming from the Echo's Space.
Per the phrasing of the Manifest Echo feature (emphasis mine):
So it's still you, prone, making the attack rather than the echo. All that is changing is the location that the attack originates from.