1) No, that's only buying spirits at chargen (don't do this).
2) No. But it would be a houserule to apply it in general anyways.
3) If you assume max services on Bound spirits is Summoning skill, and you want to be a bound-spirit-based mage, here's how you do it most effectively:
Elf.
Voodoo.
5 Willpower, 5 Magic, 7 Charisma. You don't particularly need any other stats, and I do not recommend overspending on them. You need skills more.
As soon as possible, get the Invoking metamagic (you should probably spend your first karma on joining a magical group if you can, then initiate 3 times for Centering/Invoking/Channeling).
The usual restricted gear for a force 4 power focus etc etc. I'd likely skip out on any other foci and just rely on Possession for combat, because you will have BP-shortage like whoa.
Get a mentor spirit that gives you a bonus to Guardian spirits.
Skills-wise, I'd actually take Summoning 4, Binding 6, Spellcasting 4 still. I actually think Spellcasting 6 instead is sensible too but it depends what you want to focus on.
How does this work? Invoking and Possession are what makes this fly. Why? Well, first, even low-force possession spirits are extremely effective if you stick them into a good vessel - like a dead, cybered-up troll, or a living cybered-up troll. This is because they add their Force to physical stats...and even if they have low force, if the starting creature had good stats, they are still very nasty. Invoking, meanwhile, doesn't make binding more expensive but it does make spirits way more dangerous.
The upshot is that you can get a lot of mileage out of modest-force (probably 3) Invoked spirits. If you're good at Binding, you can cough up some really, really crazy powerful great form Guardian spirits. Cycle through new spirits until you get a whole truckload of hits on Invoking, and then keep that one around (and keep re-binding it when it gets low). And with Force 3 spirits, it's only 1500 nuyen. 375 nuyen a service isn't really that bad.
Also, if your GM allows custom lifestyles (Runner's Companion), the one that gets you an aspected domain in your home is pro.
Assuming you have Magic 5, Binding 6, Guardian spec, Guardian mentor, the power focus, and the aspected domain, you can Invoke a force 3 guardian spirit with 24 dice. That's 8 hits on average..and since you aren't really taking much drain it's safe to only rebind ones that get, say, 10+ hits on that. Because of how possession works, if you stick it in an ork and give it armor and weapons, it's plenty capable of holding its own.
Because Invoking gives extra Optional Powers, you can actually get away with force 2 or even 1 spirits mechanically, although they do have crap mental stats. Your GM may screw with you if you rely on force 1 spirits to do anything but really simple tasks, maybe you can get away with force 2, but you can save lots of cash this way too since most of your power is actually coming out of Invoking, not the base force.