n2t

So, I've owned maybe 2 bikes in the last 15 years. A walmart gmc denali and a trek marlin 5.
I moved out of town onto dirt roads and my knee is a bit messed up from years running and weights. I have mild cartilage damage but nothing worth surgery, just can't run 5 miles a day anymore, biking is fine however. I'm 5'6 and picked up a Trek Marlin 5 at 15.5" sizing with the 27.5" tires. It feels pretty good, I may have some minor fitting and adjusting as my riding form progresses but I think over all I'm on a bike that fits. There are however some issues I'm noting and I would like input on.
1, grips, my hand cramps up after about 2 miles of choppy road. The palm swells seem to be on the end of the bar instead of in the middle where they would help more..should I get the extension bars and switch to grips with the extra meat in the center or just one of those options?

2, pants, I keep hooking the sprocket, but wearing shorts isn't usually a good idea as I ride in brush and we have snakes, even jeans will offer some protection against snakes..anything to do to not die, a wrap or mtn bike pants?

3, the fork sucks. I hit a large rock and it compresses and doesn't recover before the next large rock, feels like I'm on a pogo stick...suntour upgrade to something with lockout?

4, last question, my tires are the stock marlin 5 tires, I ride on loose dirt roads, grip isn't great to say the least. When there wear out, is there a better tire for this type of riding, or am I just on an unforgiving riding surface?

Thanks in advance guys. As a moderate rider who isn't going to be taking pro courses and just wants a commuter he can ride from the house, the marlin 5 really fit the bill. The 3500D wasn't a bad option either but the 5 went on sale for 450 and I couldn't resist. Hope I made a good call. I know it's entry level but I'm ok with that as long as it holds up to 2k miles a year or so.