Hello pythoneye2,
the USB magnet solution are really nice and strong.
The circle connector attach very well with force and it does not have orientation. For cons I can say that it is just for powering and just a bit bulkier than the other.
The connector oval/rectangular is with all the pins (allow data transmission), it is just a bit smaller, but it is a bit less strong and to attack correctly you need to respect orientation (cannot attach upside down).

Yes, it is, the spacer makes dark angles (what I stated as tunnel vision). Not nice.. It is reduced a lot if you limit dimensions (the spacer thickness). You can test yourself, if the battery is not to much bulky and the electronic is slim too, the effect is barely visible.

The magnet acts on the magnetic switch (that normally is on the battery cover. It is the same magnet that keeps the table-holder closed in side. If you install a magnet the MMC is powered on, if you remove it, the MMC is unmounted and powered off.
In that moment, I leaved the magnet fixed with bi-adhesive tape.
Therefore each time I open the cover, or it fell apart the MMC remained powered on and working. This is dangerous if you want to hot-remove the MMC (I mean when the phone is on): it can potentially corrupt/destroy the MMC.

Almost al phone battery must have the protection. I think there are no unprotected Samsung battery: I remove it (pretty simple but keep eyes open to do not short circuit the battery). And remember to have a B-plan in case the battery break, during these kind of operation. Stay far from inflammables elements, no water (reacts with lithium), fire-extinguisher is a good add-on, but I admit I do not have.
I must say that some battery gone crazy, heating for an internal shot circuit, but they were low cost Chinese batteries. When happen keep dissipating the heat and after discharged they became safe (just safe to trash unfortunately).

Correct! That battery was 1500mA and yes a good size, the phone last a couple of days of very intensive use to me.
The battery was soldered, there is no better way to use unprotected battery. Remember to execute fast soldering junctions. If you are slow you overheat the battery, and it can damage, apart that it is risky..
And, use brand new and similar quality battery when used in parallel to assure good and safe results.

When I will have a real home, I will buy a 3D printer to print the spacer.. but till then maybe all the N900 phones will be dead..

Otherwise much better the magnetic connector.