Learn Farsi: در خیابان دیدمش؛ صورت او مانند توپ گرد است. - I saw him in the street; his face is round like a ball.

It connects using the ezafe vowel 'e', making it spoken as mânand-e tup. Prepositions of similarity like مانند (mânand), مثل (mesl), and شبیه (shabih) need the ezafe to link to the noun or pronoun that follows them.

Could I swap مانند for مثل (mesl) or شبیه (shabih) here?

Yes, you can say مثل توپ (mesl-e tup) or شبیه توپ (shabih-e tup). They all mean exactly the same thing here. مانند is just slightly more formal and often used in writing, while مثل is extremely common in everyday conversational speech.

What is the ش at the end of دیدمش (didamash)?

It is the attached object pronoun for 'him' or 'her'. Instead of writing the separate object phrase او را دیدم (u râ didam, 'I saw him'), Farsi speakers often attach the pronoun directly to the end of the verb. Here, دیدم (didam, 'I saw') plus ش (ash, 'him') makes دیدمش (didamash).

Why does the adjective گرد (gerd, round) come right before the verb instead of right after the word for face?

Because it is the main point of the sentence: 'His face IS round'. If you used an ezafe to say صورت گرد او (surat-e gerd-e u), it would just mean 'his round face' as a single noun phrase. By placing گرد at the end with (ast), it acts as the predicate, leaving the simile ('like a ball') safely in the middle.