On your crushed/split filter end, seal rips with some type of tape and should work fine. The gaps under the filter as it sits on the filter holder may be due to the filter getting possibly getting bent in shipping? My filters are held down by a couple of spring wires that crisscross across the top of the filters.
If your gaps do not disappear once they "settle in", I will suggest getting some wide painters tape(low adhesive) to run along the filter and holder to seal the gaps. On areas of the filter that are hard to reach once filter is in place, I will suggest placing tape on those areas of the filter before you set it in place. Place the filter in place in hoping the tape will settle down to seal and then place tape around the filter edges you can reach and see there is a gap. The painter's tape is the key so that you don't have adhesive goo remaining when you change the filter the next time.
I think the best filter for my ac/heating unit costs in the low twenty dollars for the highest rating filters.
On your stand alone filters, my guess is on at least one side there is a filtration paper. On the inside will probably activated charcoal nuggets. There may be some molecular sieve pellets to remove some humidity?
I use to maintain scuba diving air compressor filters at a dive shop where I worked. The filters used cotton to filter air particles, molecular sieve to remove moisture and activated charcoal to remove volatile organic compounds (VOCs), odors, and other gaseous pollutants from the air. Periodically, we would receive a laboratory test kit to take air samples from the compressor and also the ambient air. These samples were sent away for analysis. The filters performed very well. Our compressor was near a shopping center parking lot and about 300' away from major streets. The pollution created by the cars was filtered out by the compressors filters.
I think your ac/heating filter will work. Sometimes the size filter I needed was not in stock. I took a filter that was one inch longer than what I needed and cut two slits one inch apart in the cardboard frame on both length sides of the filter. This permitted me to collapse the filter down by one inch in length by pushing the ends together. The filter seemed to work fine.
A concern on filters is that it needs to allow a certain amount of air to flow through it so that the ac/heating unit can perform as designed and not cause your fan unit to work too hard. I do not have the knowledge about what these limits might be. Originally, I use to use the common fiberglass fiber filters. I just put the higher quality filters in and observed if they worked okay. They have.