There nothing definite one way or the other. The windows were always there, but no indication what was behind them.
My unsupported belief is that the TenForward lounge was there from the time the ship was commissioned, and Guinan was aboard during Encounter at Farpoint.
It and she were just unseen.
I always assume Ten Forward was there from construction, but pretty certain Guinan wasn't. I can't remember if it is mentioned that she came on board after Yar's death, but she did say they hadn't met.
In Redemption II, Guinan talks with Picard about Sela and shows insight into her and how she can be Tasha's daughter. Picard says that Tasha had died "a year before you came on this ship" so that pretty much establishes Guinan came on the ship well after Tasha's death. Allowing for Picard to be hyperbolic, rounding up, or just exaggerating it could have been months and not really a full year.
Tasha's final episode "Skin of Evil" has a Stardate of 41601.3 and Season 2's opener, "The Child," is 42073.1.
A change of 472 "stardate units." If 1,000 stardate units represents a single solar year -there little indication it does, though most mentions of time passing seems to suggest that it does. Then 2.7 stardate units comprises 1 day. This means 174 days passed, or between 5 and 6 months, between Tasha's death and "The Child." If I'm not too much mistaken there's some implication in "The Child" that Guinan is new on the ship, her conversation with Wesley as he ponders his fate out the windows, suggests they've never spoken before and Wesley acts as if he doesn't know her.
So, given around three years pass between Guinan coming on board and the events of the "Redemption" two-parter it's probably okay for Picard to fuzzy up 6-months into "a year" to talk of the time that has passed between Yar's death and Guinan's arrival.
As noted above the original designs for the ship actually had the saucer-rim only one deck thick, but this was slightly retconed -behind the scenes- to effectively make it two-decks thick. I believe this was done without too much altering the size of the saucer section, just the thickness of the rim now being two decks rather than one, but the saucer isn't one deck thicker. Things were just shifted around. Fuzzy math and fuzzy architecture may have been used.
There wasn't meant to be a lounge like 10-Forward, it was assumed there were many lounges throughout the ship. But it does make sense there'd be something of a "central point" everyone would go to and the room with the best view would make the most sense. But in reality, 10-Fwd probably should have been a lot bigger given how many people the E-D can supposedly accommodate.
The idea was for this rim of the saucer to be one deck-thick and not have any rooms in it. Rather, it was basically one long hallway of sorts with seating, tables and other things of that sort. It was supposed to be dark to allow for a better view out the windows (and, also, because the limitations of the model precluded the saucer rim from being able to be lit) with the windows on the top and bottom separated by the wall segment which was supposed to be the sensor strip on the outside and either a blank wall, or maybe some-sort of data/stellar-cartographical information on the inside.) I think this idea was considered being put into use when they were allowed to build a new set but it was felt the windows couldn't be used dramatically enough for the show. The lower window in most shots would be out of frame so otherwise useless from a TV standpoint and this would make the top window smaller and not give as much of dramatic view with the streaking stars at warp and for various things happening out the ship. It was felt a large set of windows would be more visually interesting.
So the deck-plans were rejiggered. Unfortuantly the model being used didn't have a set of windows on the front that matched 10-Fwd, creating a bit of a discontinuity. When the smaller 4-foot model(s) was built to allow for easier filming it was built with a slightly thicker rim and with the appropriate type of window arrangement to allow for a 10-Fwd, as well as them being lit.