From what you tell us here, which isn't much, I would say that all three items could be a problem with or without the geo-grid. 1)Planting trees above a retaining wall is not a good plan unless they are set back at a distance that would also be beyond the geo-grid 2)Setting fence posts after the fact will be way more difficult than doing it as the wall goes up. 3)Changing landscape shape AFTER you have built a substantial (I am assuming this because of the geo-grid) retaining wall is not sound thinking and you should decide on form before you build.

Yep geogrids cause problems? You could lay a preseeded wildflower mat over the top, looks like carpet to begin with but if well looked after grows well.

Why are you not replacing retaining wall, you could use a gabion basket structure that can have the top third filled with soil and planted up. I have some examples of this if you are interested?

watso75
www.landscape--design.blogspot.com

Let's start with a basic question:

Do you know what Geo-Grid is and how it is installed?