"Election interference" is not a crime or a legal category. It is a term often used by the press to indicate a variety of actions, some illegal, some legal but argued to be improper or dangerous. For example, it is perfectly legal for anyone, citizen or alien, to make statements about the election or the candidates, even if these are knowingly false, as long as they do not rise to legal defamation (and the bar for that in an election context is very high). But doing this has been called "Election interference" in the press.
It is generally unlawful for an alien to make a direct campaign contribution. The Federal Election Commission page on Who can and can't contribute says:
Campaigns may not solicit or accept contributions from foreign nationals. Federal law prohibits contributions, donations, expenditures and disbursements solicited, directed, received or made directly or indirectly by or from foreign nationals in connection with any election — federal, state or local. This prohibition includes contributions or donations made to political committees and building funds and to make electioneering communications. Furthermore, it is a violation of federal law to knowingly provide substantial assistance in the making, acceptance or receipt of contributions or donations in connection with federal and nonfederal elections to a political committee, or for the purchase or construction of an office building. This prohibition includes, but is not limited to, acting as a conduit or intermediary for foreign national contributions and donations.
There is an exception for the holders of green cards (who do not fit the conditions of the question). This same site's page on Types of contributions also says that: "The entire amount paid to attend a political fundraiser or other political event or to purchase a fundraising item sold by a political committee is a contribution" regardless of the expenses of the committee or market value of the merchandise.
Thus when the question says that "purchasing election merchandise is illegal" it is correct if that merchandise is being sold as a fundraising effort, as most campaign merchandise is. But when it refers to "discussing elections with a US citizen" that is not only not illegal, it is protected activity under the US First Amendment (freedom of speech), which is not limited to US citizens or residents.
If a statement or electioneering communications is published, and its publication is paid for by a foreign national, and this is done at the request of or is suggested by a candidate or campaign committee, the payment will probably constitute an "in-kind contribution" and thus fall under the law against contributions by a foreign national.
The above is a matter of federal law and Federal Constitutional rights, and so applies in every part of the US, including CA. Any state law purporting to make an alien "discussing elections with a US citizen" unlawful would almost surely be held unconstitutional and void on its face, although I don't know of a case where there was a court decision on this exact point. A law requiring a widely distributed statement supporting or opposing a candidate in an election identify its sources or sponsors might well be constitutional, by analogy with the "I approve this message" requirement for broadcast advertisements.