MO Theory: Why do hydrogen and lithium bond, but hydrogen and helium don't? I've been cracking my head trying to make sense of Molecular Orbital Theory, but all sources I find are simplistic {If anyone has an online course or a book to recommend, I'd really appreciate}. Therefore, my questions are quite simplistic:

1 - Why should atoms in the first row {Hydrogen} interact with atoms in other rows  {Lithium, Beryllium, Boron, etc}? I mean, as a way to answer why two Hydrogen atoms form a Molecular Orbital in the first place, I have the postulate that it is due to Pauli Exclusion Principle: as the 1s Orbitals - and, by extension, its electron - overlap, a new orbital is formed to avoid* having two electrons with the same quantum numbers. But, when Lithium approaches Hydrogen, there is only a significant overlap between 1s and 2s Orbitals. In this sense, there is no threat* to Pauli's principle, given that the approaching electrons do NOT have the same quantum numbers. Which yields the question: how do Lithium and Hydrogen bond? 

. * I understand that the universe doesn't care about what we, smart monkeys, think of it. It doesn't see things as "threats". Still, interpreting things this way makes everything easier.

2 - Why won't Helium bond with Hydrogen? The approach of He and H should form bonding and anti-bonding Orbitals, which will be filled with three electrons. This should place two electrons in the Sigma Bond, but only one in the Sigma* Bond. Therefore, the bond order is 0.5! So why HeH isn't observed? 
Another way to phrase it: If He2+ can exist as a molecule, why can't HeH?

Thank you very much for your time.
Sorry if I messed something up.
You've asked seemingly simple questions that get into quirky considerations. A molecule of HeH would very unstable:

$\ce{2HeH -&gt; 2He + H2}$

As Oscar points out HeH probably can be made. In all of deep space where molecules could go days before colliding with another particle there must be at least one molecule of HeH. So it can be "made" and it does "exist." But how would you detect it? 

Typically when we are talking about "making" something we are talking about making it in a earth bound laboratory. You make enough to at least observe it somehow. For instance a lot of radioisotopes have half-lives less than a second which really doesn't give an experimenter time to do any really "chemistry" with them. So in any practical sense the HeH molecule doesn't exist.