First of all, welcome to the game and thanks for picking Solum! It’ll be great to have a new neighbor around : ) As for your questions:
1 - As @Wormate pointed out after 12-13 hours any land that has been modified in a world will regenerate itself and become like-new if it remains untouched by players. So the mountain pass you made will go away unless you claim it with plot markers. As you explore Solum you’ll find plenty of tunnels made by players that go through, over and under mountains! This is true on all of the planets. It’s a first-come, first-served type of game, but the worlds are so huge that right now there is really no need to worry about encroaching on too much space!
2 - Yes, with very few exceptions (I think the campfire is the only one I can think of) any item you place you can essentially pick back up by breaking it with a hammer, shovel, etc. This includes flowers, blocks of stone, dirt, sand, etc., as well as, all crafted items. So don’t be afraid to try something and if you don’t like it…break it down! If you do this too many times in the same physical space the game will flash a warning about you not being rewarded XP for placing and rebreaking an item over and over (preventing a player from essentially leveling up without having to move).
3 - If you are short on resources, but want to use better tools you have a few options! First off, there are plenty of player shops around selling items. I know at my location, for instance, the items are geared towards newer players. Lower prices, entry level items (up to silver), etc. You could pick up that one fossil you need for a few Cs and make that iron tool. When I go mining I try and repay the tool cost while mining. So if I’m using an iron hammer I try to get at least 10 fossils and 10 pieces of iron while using it. This covers the cost of making a new hammer, plus, the cost of creating 4 other iron tools (axes, grapples, shovels, spanners, etc.). This is VERY easy to do.
Also, the quicker you get BULK and MASS production the faster you will save on item costs. For instance, iron tools cost 2 iron bars, 2 fossil extracts and 4 sticks per tool if you make 1 at a time. To bulk make 3 of them costs 10 sticks, 5 iron and 5 fossil extracts (a savings of 1 fossil and 1 iron (16%ish!). To Mass make 10 of them costs 29 sticks, 15 iron bars and 15 fossil extracts. A savings of 5 iron bars and 5 fossil extracts (25%!).
I hope this helps!