New day, new question: I'd like to use 4 characters max - one of them being dual class fighter thief with only the bare minimum of thief levels to open any lock/disarm any trap... no stealthy back stabbing this time around. How many thief levels would I have to give him before dualclassing to fighter? I could test this with help of console but I still need to know how much points in open lock/disarm trap is enough to take care of everything in the game... My rough estimation is about 3-4 thief levels to boot, would that be a big hit to his subsequent fighter carreer? Is it better to just multiclass?

Cheers.

Go dual if you want to make just a lockpicking fighter.

As for how many levels, I don't know how many sneak points you get per level but I remember that when I tried the same, I ended up 110/120 pts at one level up. So I had to either level up again to fill it (nah), or give up maxing it (better, there's equips for that).

But, how about alternatives? I think a warr\mage with the locks opening spell would be less good at mass lockpicking but overally much stronger.

You don't need any thief levels for opening locks, everything except 2 optional locks in Lonelywood (both can be opened with the knock spell, one of them even with a strength boost priests offer) can be bashed easily with 18/00 strength.
So you only need some trap disarming skill, 2 thief levels should be enough to get 65 with the cat's grace spell (20 dex, buy the scroll from Orrick in chapter 1) which might be enough, you can take a 3rd thief level if you doubt this. If you don't want to bash locks you need 2 extra thief levels.
You can buy an item which raises thieving skill a little from Orrick in chapter 1.

If you start as thief you loose some hitpoints compared to pure fighter and cannot start with 18/00 strength which is a big disadvantage until you find the 19 str girdle in chapter 6. Afterwards you won't have a disadvantage in offensive power any more, the weapon skill points you gained as thief might even be advantageous.

I'd still need to disarm traps somehow... regardless how little harm they would do to me I dont like just walking over them from roleplaying standpoint :P Things aren't looking up however... at thief level 4 both my open lock/disarm traps are only in 60's (20 pts per level)... I dont suppose thats nuf? Progressng in thief beyond lvl 4 before dualing to fighter seems too risky to me. At level 6 I reach 85/80 maybe that could be nuf but taking 6 levels of thief seems bit drastic to me...

I guess up to 10k points w/o thief would be bearable but 7 levels w/o one seems too much to me even with the alternatives the present themselves - find traps spell, knock etc. On the other hand 7 levels would give me that evasion (nice call on that) and also allow me to boost stealth so he would also be usable as a scout since there are plenty of items that boost stealth. This is what I love about rpgs... dilemas and decisions. If this was bg2 which I finished 7 times I would know exactly when I will reach what level but IWD with HoW and TotL is totally new to me. Needless to say, I'd like to avoid scenario where I would be struggling for better part of the game and only reaped the rewards in last few moments...

It's better to run that character as a multiclass fighter/thief, they level up pretty quickly. Since you want a fighter that can disarm traps, make him/her a dwarf since they can get 19 CON, bonus saving throws, +10 to Open Locks, and +15 to Detect Traps. Dwarves can only get 17 DEX, but that's a small price to pay. You could also use a gnome, who can get 18 DEX, the saving throws that the small races get, the bonuses to the lockpicking and disarming skills and can wear a kick-ass helmet that only gnomes and halflings can use. Halfings can get 19 DEX and have the highest thief skills at the start, but can only reach 17 STR, which really hurts and is probably not recommended for your build.

Keep in mind that you can't use thief skills if wearing armor heavier than leather, which leads to the amusing situation of having to take off your armor every time you need to disarm traps or unlock doors. (You could just bash them open if he has 18/xx Strength, which you should). It's really not a big deal since you can pause and put the armor back on should your character encounter some monsters.

I tried to level up dual class and multiclass parallelly and the first time the dual class overtook the multiclass by two fighter levels was at 1040000 xp points... that time the dual class was 7/12 (T/F) and multi 12/10... From that point on the dual class started increase the lead and at 2040000 xp it was 7/16 for dual class and 14/12 for multiclass, big difference in thac0 and significant difference in saving throws in favor of dual class. The question is, what level can I realistically expect to reach with party of 4.