Don't worry, those letters are not part of the standard orthography of Interslavic. Although some people insist on using them, they are merely meant for educational purposes, and to make a direct link to Old Church Slavonic. The only non-ASCII letters in the Latin alphabet that are mandatory are Č Š Ž and Ě (although in Interslavic nothing is mandatory).
And of course, Interslavic can be written in Cyrillic as well, as is done by many users.
Modern Hebrew is just one example of the fact that there is no such thing is a binary distinction between natural and constructed languages. The main difference between both extremes is that a natural language is the result of a process of gradual evolution, whereas a a constructed language can be defined as "any language that is based on deliberate invention, modification or selection of the core material by a concrete author or group of authors with a discernible purpose in mind".
However, every natural language has invented elements and every constructed language has naturalistic elements. Some languages that are usually considered natural display an exceptionally high level of human intervention, and some of them even have an author. Other examples are Old Church Slavonic, Nynorsk, Bahasa Indonesia, Revived Cornish, Katharevousa Greek, Rumantsch Grischun and Euskara Batua (Standardised Basque).
All we can say is that these languages, along with languages like Interslavic, Interlingua, Latino Sine Flexione etc. belong to a gray area between natural and artificial ("semiconstructed languages").
The main problem with Russian is that Russia/the USSR have been trying to impose it as a lingua franca in Central and Eastern Europe, and failed. Most people in those countries simply won't accept it.
Besides, Russian is a complex language, with lots of elements that are different from other Slavic languages. Most people who haven't learned it won't understand it either.
There is a critical difference between Esperanto and Interslavic. Esperanto is meant for communication between Esperanto speakers. Interslavic, on the other hand, is meant to be understandable for Slavs who haven't actually learned it. In other words, the success of Interslavic cannot only be judged by the number of people who learnt it. Basically, one speaker is enough to serve an entire audience, one translator is enough to serve all Slavic readers. Creating a community of Interslavic speakers has never been the primary purpose of creating the language.