The short answer is that we can never know because it did not happen that way. However, it is always fun to speculate, as long as we know we are playing games.
Your point 1 is the more difficult one to try to think through. It certainly makes Little Round Top less important if one cannot observe anything from that point. On the other hand, the hill would still make a good defensive position. Although trees would give an attacking force some cover, they would limit its ability to form good battle lines. Probably the result would be much the same as at Culp's Hill, where Confederate forces that outnumbered the defending Union forces were unsuccessfully attacking at the about same time (or just a little later) as the actual fight on Little Round Top.
Whether Little Round Top is wooded or not, Gen. Sickles decision to advance and not anchor his left on that hill would likely not change. One might argue that a wooded Little Round Top would not have allowed Gen. Warren to see Gen. Hood forming for the attack. But Warren was sent by Gen. Meade to check on the left of the line precisely because Meade was concerned about it once he learned the Sickles was out of position. The difference would be that Warren would have gone to Houck's Ridge instead, where he would be able to see to the south and could have observed Hood's men on Warfield Ridge. It may have been that Col. Vincent would have advanced to Big Round Top rather than stopping on Little Round Top. Either way, Vincent's Brigade would likely have succeeded in stopping basically the same Confederate forces, who would have been just as tired from a long march and counter march. Also, fighting in the woods of Big Round Top would be no different than fighting in the woods of Little Round Top, which is where much of the fight between Vincent's Brigade and elements of Law's and Robertson's Brigades engaged. For that matter, the fight may have taken place on Little Round Top in any event. (see below regarding timing.)
In your point 2, Big Round Top is clear (I assume on the west side). The battle would have played out in much the same way but the Union Signal Corps would have put its post on Big Round Top, rather than Little Round Top. Arguably, Vincent and later the 140th NY under Col. O'Rourke might not have arrived in time because they would have had longer to travel to get to Big Round Top and the Confederate forces would have arrived at Big Round Top to engage sooner. However, in my view at least, Vincent's Brigade would still have been able to successfully defend on Little Round Top, at about the spot where they did fight. That is simply where the timing would have brought them together. The thrust of Hood's attack would still have been targeted at Houck's Ridge, where they had a serious desire to capture and silence Smith's Battery.
Even if the Confederates could have advanced artillery onto big Round Top with a clear field of fire, it would not have been able to reach nearly as far north along the Union lines as it could have if it occupied the open ground on Little Round Top. Having said that, Confederate Artillery on Big Round Top with an open field fo fire could have proven most effective in helping to clear Houck's Ridge and the Wheatfield with far fewer infantry casualties. That may have put Longstreet in a better position to carry out Lee's original orders for the morning of July 3. The importance of Big Round Top being open is that once Confederates (15th Alabama) occupied it, the artillery platform would have reduced Longstreet's Day 2 casualties, and perhaps have affected how Day 3 played out.
However, as noted above, we can never know.
Bringing the