After Boldwyr Indimidator's ability resolves, the creature is only a Coward, and not any other creature types, for the rest of the turn.
This situation is covered in the rules that describe the interaction of continuous effects. In particular, the following rules are relevant:
613.1. The values of an object’s characteristics are determined by starting with the actual object. For a card, that means the values of the characteristics printed on that card. For a token or a copy of a spell or card, that means the values of the characteristics defined by the effect that created it. Then all applicable continuous effects are applied in a series of layers in the following order:
- 613.1d Layer 4: Type-changing effects are applied. These include effects that change an object’s card type, subtype, and/or supertype.
613.2. Within layers 1–6, apply effects from characteristic-defining abilities first (see rule 604.3), then all other effects in timestamp order (see rule 613.6). Note that dependency may alter the order in which effects are applied within a layer. (See rule 613.7.)
In addition, rule rule 205.1a describes how type changing effects like the one on Boldwyr Intimidator work:
Some effects set an object’s card type. In such cases, the new card type(s) replaces any existing card types. Counters, effects, and damage marked on the object remain with it, even if they are meaningless to the new card type. Similarly, when an effect sets one or more of an object’s subtypes, the new subtype(s) replaces any existing subtypes from the appropriate set (creature types, land types, artifact types, enchantment types, planeswalker types, or spell types). [...]
So, when determining the type of the creature that is affected by Boldwyr Intimidator's ability, first you apply the Changeling ability, because it is a characteristic-defining ability. This gives the creature every creature type. Then you apply the Intimidator's ability, which replaces its set of subtypes with just the type "Coward". So, in the end, it just has the subtype Coward.
Regarding the example, you can see in the interaction of continuous effects rules that layer 7 is "Power- and/or toughness-changing effects are applied." These effects, including Muscle Sliver's effect, are applied after type changing effects, so by the time you go to apply them, the Changeling Sentinel is only a Coward, so it is not buffed by Muscle Sliver's ability.