Yes, the design tool seems to be broken, at 58 km you are looking at a fresnel zone size of 3000 m, which at 20 m pole height just isn’t going to work. It seems that Mimosa’s design tool doesn’t account for water at all which you need to avoid interacting with for a reasonable and steady signal. The tool is saying you have LOS, but Line of Sight also requires area around where your signal is traveling to be unobstructed as well. FYI, the Mimosa design tool, while a very useful and good tool, is not a replacement for a real RF Engineer for long links like this. Also you can look at the picture and see that there is water in the path of your link.
This is what you should be looking for:
Playing around with the tool you will not have a clear Fresnel zone until you have a tower that is +75 meters tall, on both sides. Even at that height you will have issues with thermal ducting/inversion which will plague your link with signal fluctuations.
There are several different options for dealing with that issue, one is to build a diversity link which will utilize multiple antennas to keep signal strength good when the signal gets bent and reflected. (I linked to Mimosa’s Blog post about it in my last post) Another is to use an antenna with a very wide beamwidth, but that only works over short distances. You can also attempt to go over the reflections, my understanding is that with really large structures you can avoid thermal inversion/ducting altogether, but you have to be really high up. Finally, the solution taken by major telco’s in the USA was to go around large bodies of water. (Basically they would have relays of towers and short links to get around the issue.
None of these options are cheap and the first two require that you have a real RF Engineer work with you on the link.
I have never asked Mimosa support for RF Engineering help, but you might contact them and see if they have anyone who could help. But I have never heard of that, so I don’t know.
To put it simply, you need a much taller structure to mount to on each side at a dead minimum. But you will have all sorts of issues unless you deal with the thermal ducting/inversion issues I have been talking about. To be straight with you, I would not build this link period, you will have lots of problems and erratic behavior.
Per your antenna questions. If you were to use the C5c then yes both antennas will work with those radios, the B5c will need adaptor cables because it has N Type connectors and both of those antennas use SMA type connectors (which the C5c has).