Karen, here are some thoughts on your questions.
1. Every bus I have tried to board has had either a wheelchair ramp or a wheelchair lift. Every transit agency I have checked about (my checking is limited to the US) has claimed every bus in their fleet has either a wheelchair ramp or a wheelchair lift. Most transit agencies I have checked into also have a van service for those unable to use a bus for some reason, though the requirements to be able to take advantage of this service varies from strict to loose.
My two C500s fit easily on every bus I have tried. I go up the ramp/lift, make the turn down the aisle, and then pivot into a space created by folding up one of the forward seats. For some reason I find that easier to do on the driver's side vs the passenger's side, but can do either without much trouble.
2. Wheelchairs are allowed anywhere on federal lands unless specifically prohibited. While motorized vehicles are prohibited in wilderness areas and off sidewalks and roads in national parks, a law was passed (at the federal level) designating wheelchair uses as pedestrians/hikers, so we are allowed anywhere not specifically prohibited.
The requirements are that the wheelchair must have 4 wheels and must be suitable for indoor use. The only places I have seen prohibitions are for wheelchairs with 2 wheels (adapted segways) and that is for areas that would be dangerous if power to the stabilizing system was lost. This is the case on paths and boardwalks near thermal features in Yellowstone. A dip in one of the thermal features would be fatal.
States seem to have followed the federal lead, but I don't know the rules for every state.
Where I live, and everywhere I visit, wheelchairs are allowed on bike paths. I have never had anyone give me a hard time about being on one. In fact, most folks are excited to see me out and about in my wheelchair.
In short, yes, wheelchairs are permitted on bike paths in the US. I have ridden many hundreds of miles on bike paths.
3. I can buy snow/mud tires via NuMotion. They charge about $800 for the pair of drive tires. The reason is because they don't want to mount the tires on the rims, so they buy them from Permobil with rims, tubes, and tires all mounted up and ready to go.
I persuaded NuMotion to sell me a set of Permobil snow/mud tires alone for about $200 shipped. My friend and I mounted them with some tubes (from Amazon) on the rims that came with my wheelchair.
They are much cheaper from other sources, though I have only found Permobil branded snow/mud tires. The price seems to be about the same (about $100), but the shipping varies dramatically (from a few dollars to about $100!).
I have acquired two spare sets of rims. I pay about $50 to $100 per pair on ebay. They are over $500 from NuMotion.
There are some lessons to be learned about removing the regular, foam-filled tires as well as mounting the pneumatic snow/mud tires. But, once those lessons are learned, it is not a hard job at all. Anyone who has changed a bicycle tire will feel right at home doing this.
We jack up the wheelchair either with an automotive floor jack or using leverage with a four foot long 2x4 (blocking the wheelchair up with wood). Be sure to chock the wheels first.
To remove the wheel, for allen-headed bolts are removed and the wheel just about falls off. The wheels use spit rims with an outer half and an inner half held together with 6 bolts. If pneumatic tires are mounted, bleed the air out before removing those bolts.
We use steel-cored tire irons I picked up at Performance bike (their house brand...they have an orange plastic exterior). The are excellent. Inferior tire irons will break.
Using leverage rather than brute force, we remove the tire from the rim. To put the new tire on the rim, slide the tire on the larger split rim, inflate the tube a bit, slip the tube on the rim in the correct orientation (if using a tube), and lightly place the other half split rim in place aligning the bolt holes and valve stem cutout.
Before bolting the split rims together, use something like a popsicle stick to push on the tube and make sure it will not be pinched by the split rims. Then bolt the split rims together, tightening gradually in sequence around the rim.
It is much easier if you buy 3 longer bolts for the rims. You can use these to get things started, then put in three of the regular bolts, remove the three longer bolts, and finally install the final three regular bolts. There is a youtube video showing how to do this.
I have done this entire job before, but am no longer able to do so. My friend Greg helps me out with wheelchair maintenance and we have a great time while doing it. It is kind of like working on bikes
.
I like Kenda tubes and bought Kenda Inner Tube - 3.00-8 - TR-87 Valve Stem 71505622 from amazon. They are a perfect fit and heavy duty.
Two other tips. Don't forget to use locktight where required (called out in the service manual). You want locktight blue, NOT locktight red (which is permanent unless heat or solvent is applied). Confusingly, locktight red comes in a blue bottle, so be careful when buying it.
Second, I highly recommend getting a torque wrench. The service manual provides the torque setting for many of the bolts on the wheelchair. I think it is particularly important when working on safety critical bolts, such as those on wheels. I bought a TEKTON 24330 3/8-Inch Drive Click Torque Wrench (10-80 ft.-lb./13.6-108.5 Nm) from Amazon and am very happy with it. I also have a 1/4 inch torque wrench from my bicycle days. I use it regularly for smaller bolts on the wheelchair.
When I am riding around on pneumatic tires I always carry a pump, spare tube, patch kit, and tools in a backpack. While I won't be able to repair a flat, I have confidence someone will come along who will be willing to help out.
I like Rema patch kits. My favorite portable pump is the Topeak Turbo Morph Bike Pump with Gauge (bought from Amazon). This comes with a mounting bracket that can be easily mounted on the side of the wheelchair seat's unitrack (you will need to procure two small bolts and nuts. I ride my wheelchair into Ace Hardware and they help me pick out just the right size.
Sorry this got a bit long. I am sure it is much more information than you were interested in hearing and it is far off topic from the OPs initial post. I hope it is helpful.
Steve