Yes, sammeln is for inanimate objects I would say. You would use it if you were collecting apples from the garden, or collecting stamps. You can't sammel a person.

Yes. Or „Lasst [familiar plural] uns ihn abholen.“.

‘Sammeln’ on its own means to collect a bunch of things; it can't really be used with a singular object, and it's actually rather odd that the English “collect” can be used in that way. The ‘Ein-’ in ‘Einsammeln’ adds a connotation of “inward”, like the “in-” in “ingathering” — that is, we're not just collecting items, we're bringing them in as well.

„Lasst uns ihn einsammeln.“, or „Erlaubt uns ihn einzusammeln.“.

In English, 'let us' or 'let's' is a way of expressing a command or a request in the first person plural (we) - e.g. "Let's go". In German, commands in the first person plural are expressed by switching the subject (wir) and the verb (sammeln).

So - "Wir sammeln ihn ein" = "We are collecting him" "Sammeln wir ihn ein" = "Let's collect him"

and - "Wir gehen" = "We are going" "Gehen wir" = "Let's go"

NOTE - when we use this construction, we use the subjunctive form of the verb (Konjunktiv 1), but in first person plural (we) it is exactly the same as the normal form of the verb - EXCEPT with the irregular verb 'sein' (to be) e.g. "Wir sind dankbar, dass..." = "We are thankful, that..." "Seien wir dankbar, dass..." = "Let's be thankful, that..."

(The subjunctive is complicated grammar, so you probably don't need to worry too much about it until you are quite advanced.)

Das ist nur für das Geld: as in "collect the rent".

If the pronoun ‘ihn’ refers to a person (=“him”), a bounty-hunter or kidnapper could say »Lass uns ihn kassieren.« in the sense of “Let's cash him in.”. In the sense of “collect”, the pronoun ‘ihn’ (=“it”) could refer to ‘den Gewinn’ or ‘den Preis’ = “the prize”, but not to a person.

In English, the expression “Let us collect him!” can be either cohortative, as an exhortation addressed to “us”; or imperative, as a command addressed to someone other than “us”. The contraction “Let's” can be used for the cohortative, but not for the imperative.

In German, the expression „Lasst uns ihn einsammeln!“ can likewise be either cohortative, as an exhortation addressed to ‘uns’; or imperative, as a command addressed to plural familiar people other than ‘uns’. However, the expression „Sammeln wir ihn ein.“ can only be cohortative.

The construction ‘Sammeln wir ihn ein.’, inverts the order of the subject ‘wir’ and the conjugated verb ‘sammeln’, so it's clear that the sentence is not indicative. The verb ‘lassen’ is not used in this construction because the speaker belongs to ‘wir’.

In the cohortative sense of “Let us collect him.”, English omits the subject “we” and uses the form “let us” even though the speaker belongs to “us” because English uses subject-verb inversion only for auxiliary verbs, as in the question “Are we collecting him?” or “Do we collect him?”, not for main verbs (*“Collect we him.”), but English has no appropriate auxiliary verb to fill the sentence-initial slot for a cohortative, so instead it borrows the imperative form.

»Lass uns ihn einsammeln!« is the second-person singular familiar (‘Du’) form.

»Lasst uns ihn einsammeln!« is the second-person plural familiar (‘Ihr’) form.

»Lassen Sie uns ihn einsammeln!« is the second-person formal (‘Sie’) form.

For the cohortative “Let's…”, where I'm addressing us, the familiar (‘Lasst uns’) form is always used, because we have to be familiar to me for me to call us “us”.

For the formal imperative “You all let us…!”, the formal pronoun ‘Sie’ is always included, to disambiguate the form ‘lassen’, which is identical to the third-person plural form for ‘sie’=“they”.

In this case, 'he' is the direct object and so takes the akkusativ. Er (nom) Ihn (akk) Ihm (dat) 'Ihm' would be used if 'he' was the indirect object (i.e. dative case), e.g. "Lass uns ihm es geben": "Let us give it to him".