No, it's not the same, because there are plenty of circumstances in English where mixing up R and L could lead to ambiguity or confusion.
how many? Anyway less than lack of distinction between all the Russian hard and soft consonants.
Pronouncing ся as sa or s-ya does not cause listeners any problems, and in the context of all the other mistakes and hesitations a beginner will be making it's not even going to be noticeable.
An unfair example. Both pronunciations with the hard and the soft s exist in Russian. But what for should Englishmen replace a soft consonant with a hard consonant and a "y", if it doesn't make a sound closer to the Russian one, but makes the pronunciation more difficult?
Don't be silly. There's nothing to "relearn" by skipping over this initially, you just pick it up later when you're ready, either by progressing to more advanced texts or simply through familiarity with the language as spoken by natives.
Wrong pronunciation is a bad habbit which is hard to correct.
If you learn it at the beginning, it will become natural and you will learn everything in the correct pronunciation.
Why do you think that conjugations and declensions are important? Я жить Москва is perfectly understandable. Should Russians train to pronounce voiced consonants in the word-final positions? Can't it be understood from the context that dick is dig?