No. The progressive form is more natural with a verb like "offer" describing the present (typically for all verbs except statives like "like"). Here, the first sentence actually sounds more polite, because it is more formal, and less natural. I'd tentatively say it's not really present tense at all, but a sort of <b>performative</b> (similar to "I now pronounce you husband and wife"), rather than an actual present description (e.g. "I'm pronouncing that...").

Generally the past tense (or a conditional) can be polite, but the use of progressive forms does not seem familiar in this sense, mostly because it's just natural. But politeness is always relative, and often based on what would be marked (less natural, or less basic) in a particular situation, so maybe sometimes it works like that. In the past tense it does seem to be used like that, as in "I was wondering if I could ask...", but I don't know that the progressive itself is the main factor there.

For the first point, yes, that sounds about right, and the examples from Swan seem accurate to me.

For the second point, it's hard to explain what the difference is there. As with the first, a major effect is just based on frequency and which form is more typical, so anything that is different will have some kind of pragmatic effect in context ("oh, you said something that isn't so typical, so you must have a reason for saying it differently, so you probably are intending to be... [formal, polite, funny, etc.]" = a typical implicature).

I suppose what might be happening here is that "How do you expect..." is a fairly common argumentative response of disagreement, implying there is no way to cooperate and that the other person is being unreasonable (associated specifically with this lexical verb). By using an alternative form ("How are you expecting...") you avoid that common association. It also highlights some kind of ongoing act ("be expecting"), implying forethought and a plan, so it may be a more sincere question. Compare also:
Did you plan for that to happen?
Were you planning for that to happen?
The first is similar to just asking "did you expect...", about expectations and possibly intent. The second instead emphasizes the planning, suggesting the result was not just intended, but also completely under the person's control, and with a specific goal.

There can be multiple factors, with multiple effects. It's complicated. Some of it is contextual. As I said, an important part is expectation, and the implicature generated by not saying any of various possible alternatives. The effect may be different in different contexts, and for different reasons.

As for these specific examples, yes, the progressive form is "less definite", so that could mean various things: casual/uncertain, or  also polite by being unimposing (because it's less definite/demanding). The simple present would sound a bit more formal, and possibly therefore polite, but also could sound more direct and therefore rude. As I said, it's complicated!

I suggest trying to understand relevant factors and their possible effects, instead of trying to determine "rules" about usage.

(Amusingly, I just noticed that, after correcting a typo, if I had ended this with "I am suggesting" vs. "I suggest", the progressive form could actually sound impolite, somewhat insistent, like "I've been trying to tell you for a long time....", while the simple form sounds like a new idea, and therefore not impolite. Other times, as you've noticed, the opposite is true.)

The so-called "present tense" is rarely used as an actual present tense, but instead as habitual, etc. When describing the present moment, the progressive is the typical form.* So "I'm offering..." is a normal description. As I said above, "I offer" is different, something we could call a performative, where the speech act itself is an action, not just a description. And in that case, the simple present form is used.

(*The simple present is used with stative verbs like "I know", versus "*I'm knowing" which is extremely rare as a progressive, so it's easy to tell which verbs fall in that category by their lack of usage in the progressive. The most obvious examples are common verbs like "be" and "have", although there are rare exceptions that really don't describe states but activities such as "He's being rude" or "He's having a bad day".)

To make it more complicated, though, you can compare this to usage in the past, where progressive or simple past forms can be used, with slightly different nuances.

In the end, it's important to think about potential alternatives, and what the effect of not using those alternatives is.