Keyboard with extra thumb keys (below or near the spacebar)

Question

I need a keyboard which has an extra key in the area of the left thumb.
I need this extra key to bind it to the mouse left button, because I use graphic tablet a lot and it's next to impossible to work normally without a mouse click functionality. Currently I use Autohotkey and bind the LWin key to emulate the mouse click, but it is inconvenient
because the LWin key is in the 'dead' zone and I need to make extra movements all the time so it is quite painful.
So I need a keyboard which has some extra key in the thumb area i.e. where the space bar normally is. The problem though - none of the standard  keyboards have them (they only have a huge spacebar as everybody knows).
I've spent a lot of time googling and I have found something that looks promising:

source: http://mc.pp.se/dc/kbd.html
It's a Japanese keyboard. This seems exactly what I am looking for, because there are extra buttons on the bottom row. All Japanese have them so I could buy one, BUT: the problem is, I am not sure how this extra key (left to the spacebar) will act normally if I use Windows  and e.g. standard US layout? Will it be possible to make use of it by binding it to the mouse click with Autohotkey? Or maybe there are some Japanese keyboards produced for EU/US market with those keys as a bonus (would be ideal)?
Also I have found another possible solution: ThinkPad Lenovo keyboard with trackpoint:

But I don't like it because it's not full-size, and I don't really need
the trackpoint.
One option which looks promising as well is MS Natural Ergonomic 4000:

because of two extra buttons below the spacebar. Again Not sure if they can be used to emulate mouse click.
So ideally I want something like the Japanese keyboard, but only if I can really make use of those keys
in the bottom row.
Questions:

Can I make use of that extra keys on Japanese keyboards, e.g. rebind with Autohotkey to other keys (mouse click)?
Can I bind the MS Natural 4000 extra keys (below the spacebar 'prev' button) to the mouse click?
Keyboards with split spacebar (2-piece spacebar) - can the left piece be bound to other key by any chance?
Do you know of any analog or similar keyboards, i.e. with extra keys on the bottom row that can be used in such way?

Ideally the keyboard should be easily available and not very expensive. Obviously there is something
like ErgoDox, but I am not ready for this investment of money and time yet.

Mikhail V · Accepted Answer

So, here is my "happy end" story. I could not find more good options 
so I was a bit on the fence about what to buy ... and know what? Recently I was in the local computer store and what did I see! There is a keyboard sold which is exactly what I need. It is A4Tech X7 G800V model, a budget (~30$ in Russia) keyboard with extra buttons below the space bar.

Now I am really puzzled why google was so inefficient on this when I ve been  searching for "keyboard with extra buttons", "extra key row" etc. and what not - nothing valuable popped up.
And almost by accidence I have noticed this keyboard in the local computer store.

I have bought it and have been testing it for a week, and I am very pleased, so I've 
even decided to write about my experience so far.
Here is a link to the product: 
http://x7.a4tech.com/product.aspx?id=87

image:

Note: there is another very similar keyboard, G800MU, but it is not suitable for the described needs, because it has no possibility to reprogramm the extra keys, whereas the G800V has this possibility, so be cautious when buying.

So what I get:

Fully reprogrammable extra keys - 15 keys total (gray keys marked with G and a number). 
There is a software "X7 Oscar Keyboard Editor" in the downloads and its very easy to use - no reboot 
required to reprogramm the keys. So you can reprogramm any G-key to any key, mouse button, or even some macros and more. And that is really cool. Absolute no problems or glitches with that so far.

Overall very good build quality and materials (despite first impression mainly because of childish red WASD keys).
It is a membrane keyboard, but feels really good and smooth - low resistance force and short travel reminds me of 
some 90's Chicony keyboard feel.

Now to the best feature - extra keys below the spacebar is what I needed so much: now I have both mouse buttons 
on my keyboard in a comfortable position under my thumb and don't need any software tweaks and manipulations. 
Working on a graphic tablet finally became comfortable (thanks heaven and a4tech).
So I just disable the "click on tip touch" feature in the graphic tablet driver and bind the G-7 key as left mouse button.

Very nice bonus - an extra key (G-11) near the ESC key - for now I have bound it to 0 key so I can type numbers like 10, 20, 30, 01, etc. with left hand easily.

Few minor issues that I have noticed:

Strange decision from a4tech to place the left Alt key below the Z key and to put G12 extra key on its place - this is IMO mistake, BUT - you can easily reprogramm this G12 key  to Alt and the problem is solved, that's what I did, so in 
the end my Alt is even better placed than on many modern keyboards.
The G1-G4 keys (oval keys on the left side) are really hard to press, no idea why they made it so. Definitely those could be more useful if they worked with less force.
There is no replacement keycaps for those red silicone keycaps (not a real issue though).
The legs are a bit small, I'd want to raise the angle more -- needed to put some 10 mm blocks under the legs 
to raise the angle more, but that is individual preference I think.

Happy end :-)

TommyHigginsELL · Answer

Can I make use of that extra keys on Japanese keyboards, e.g. rebind with Autohotkey to other keys (mouse click)?

Yes, I tried it myself and it worked perfectly. Below is the actual script I used. SC07B is the "無変換 (muhenkan) " key on the left and SC079 is the "変換 (henkan) " key on the right.

In case the your future Japanese key code is different from mine and you don't know how to figure it out, check how to: https://superuser.com/questions/1150394/autohotkey-remap-a-solid-key#1150396

#NoEnv  ; Recommended for performance and compatibility with future AutoHotkey releases.
; #Warn  ; Enable warnings to assist with detecting common errors.
SendMode Input  ; Recommended for new scripts due to its superior speed and reliability.
SetWorkingDir %A_ScriptDir%  ; Ensures a consistent starting directory.

;;Clicks
SC07B::Send {Click}
SC079::Send {Click, right}

FYI: Did you notice the symbols are different, like " on 2 or ) on 9? It may be confusing at first but you'll get used to it within weeks.

Keyboard with extra thumb keys (below or near the spacebar)

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