If I understand you correctly then the mercantile skill affects what vendors have in stock in addition to getting a better price?
I'm also a bit hesitant to lower my throwing skill, since the skill also affect damage done right?
I got 15 perception since I'm gonna be burst-firing a lot and the perception boost will allow me to hit with more of the bullets, but if can take 3 points out of perception and still hit with all those shots then maybe I should put them into constitution instead? So screw stealth, put 1 point into strength for metal armor, and 2 points into constitution for some more hit-points. This will also make me able to keep a high throwing skill.
Dropping stealth will also mean disarming traps only. How many skill-points do I need in traps if I'm only disarming them?
In order, then:
Some (but not all) merchants have a special stock that can only be accessed by a character with sufficient skill in Mercantile. If you're willing to see mild spoilers, then this page of the Wiki will tell you what the thresholds are:
http://www.underrail.com/wiki/index.php?title=MercantileThrowing skill does not have any effect on damage done for grenades, only for knives. Since throwing knives are really not very good weapons unless you build specifically for them (and you have not, since your Dex is so low) I would seriously encourage you to throw mostly grenades and perhaps nets, and only very rarely use knives, if at all. Thus, for your build, higher throwing skill only will improve your accuracy, not your damage - though obviously if you hit more often, you're going to do more damage. But I play with 30 Throwing on my characters, just enough to get Grenadier, and the accuracy is almost always acceptable.
12 Per will be just fine. In all likelihood you'll be wearing goggles that will give you +1 to that, so you likely will have 13 effective. Things that are hard to hit, well, that's what bear traps and flashbangs are for.
On Normal difficulty, you can probably get by with about 70 real points in Traps, if you're also going to have a Trappers Belt and the Jackknife. 120 or so should let you disarm just about everything.
If you're going to go with metal armor, then you won't need quite as much Tailoring as you have. 90 points with your Int will give you 128 with housing bonus, and that's the "magic number" you want your Tailoring to hit for making metal armor, since that's the highest you could possibly need, in the event you luck out and get the highest quality carrier vest possible.
EDIT: Ah, I missed your edit. Let's see. You will never need a Bio skill higher than 130 effective, so you can pull back some of those points. Conditioning *sounds* really really good, but the way it applies, it's at best only "not bad". My suggestion would be something perhaps more like this:
http://underrail.info.tm/build/?GQgHAwYMAwfCh8KHAAAAAABfXwBQc3MpY1oAAAAAAAA7T1MWFTBKJklLMUUkJw As you can see, this leaves you 80 points to invest in whatever you like. Also, Nimble isn't any use for your build, now. If you think you might be willing to go down to 11 base Per, you could get one more point in CON and that would allow you replace Nimble with Juggernaut, which would be much more helpful for you.
Because you have no passive defenses (Dodge, Evasion, Stealth) you're going to find yourself in a lot of situations where enemies are right in your face and you would prefer they not be. Quick Tinkering is a godsend for builds like yours. You just drop a bear trap and take a couple steps back, then on their turn they run right into it. Not only can they not stab/punch/smash you, but they're also much, much easier to hit.
