diff --git a/testing-framework/core/src/scenario/definition.rs b/testing-framework/core/src/scenario/definition.rs
index 70aa200..f2c4e32 100644
--- a/testing-framework/core/src/scenario/definition.rs
+++ b/testing-framework/core/src/scenario/definition.rs
@@ -4,7 +4,7 @@ use thiserror::Error;
 use tracing::{debug, info};
 
 use super::{
-    NodeControlCapability, expectation::Expectation, runtime::context::RunMetrics,
+    DynError, NodeControlCapability, expectation::Expectation, runtime::context::RunMetrics,
     workload::Workload,
 };
 use crate::topology::{
@@ -21,6 +21,14 @@ const MIN_EXPECTATION_FALLBACK_SECS: u64 = 10;
 pub enum ScenarioBuildError {
     #[error(transparent)]
     Topology(#[from] TopologyBuildError),
+    #[error("wallet user count must be non-zero (got {users})")]
+    WalletUsersZero { users: usize },
+    #[error("wallet funds overflow for {users} users at {per_wallet} per wallet")]
+    WalletFundsOverflow { users: usize, per_wallet: u64 },
+    #[error("workload '{name}' failed to initialize")]
+    WorkloadInit { name: String, source: DynError },
+    #[error("expectation '{name}' failed to initialize")]
+    ExpectationInit { name: String, source: DynError },
 }
 
 /// Immutable scenario definition shared between the runner, workloads, and
@@ -84,9 +92,10 @@ impl<Caps> Scenario<Caps> {
 /// Builder used by callers to describe the desired scenario.
 pub struct Builder<Caps = ()> {
     topology: TopologyBuilder,
-    workloads: Vec<Arc<dyn Workload>>,
+    workloads: Vec<Box<dyn Workload>>,
     expectations: Vec<Box<dyn Expectation>>,
     duration: Duration,
+    wallet_users: Option<usize>,
     capabilities: Caps,
 }
 
@@ -109,6 +118,7 @@ impl<Caps: Default> Builder<Caps> {
             workloads: Vec::new(),
             expectations: Vec::new(),
             duration: Duration::ZERO,
+            wallet_users: None,
             capabilities: Caps::default(),
         }
     }
@@ -145,6 +155,7 @@ impl<Caps> Builder<Caps> {
             workloads,
             expectations,
             duration,
+            wallet_users,
             ..
         } = self;
 
@@ -153,6 +164,7 @@ impl<Caps> Builder<Caps> {
             workloads,
             expectations,
             duration,
+            wallet_users,
             capabilities,
         }
     }
@@ -173,7 +185,7 @@ impl<Caps> Builder<Caps> {
         W: Workload + 'static,
     {
         self.expectations.extend(workload.expectations());
-        self.workloads.push(Arc::new(workload));
+        self.workloads.push(Box::new(workload));
         self
     }
 
@@ -205,17 +217,15 @@ impl<Caps> Builder<Caps> {
     /// Override wallet config for the topology.
     pub fn with_wallet_config(mut self, wallet: WalletConfig) -> Self {
         self.topology = self.topology.with_wallet_config(wallet);
+        self.wallet_users = None;
         self
     }
 
     #[must_use]
     pub fn wallets(self, users: usize) -> Self {
-        let user_count = NonZeroUsize::new(users).expect("wallet user count must be non-zero");
-        let total_funds = DEFAULT_FUNDS_PER_WALLET
-            .checked_mul(users as u64)
-            .expect("wallet count exceeds capacity");
-        let wallet = WalletConfig::uniform(total_funds, user_count);
-        self.with_wallet_config(wallet)
+        let mut builder = self;
+        builder.wallet_users = Some(users);
+        builder
     }
 
     #[must_use]
@@ -223,18 +233,34 @@ impl<Caps> Builder<Caps> {
     /// components.
     pub fn build(self) -> Result<Scenario<Caps>, ScenarioBuildError> {
         let Self {
-            topology,
+            mut topology,
             mut workloads,
             mut expectations,
             duration,
+            wallet_users,
             capabilities,
             ..
         } = self;
 
+        if let Some(users) = wallet_users {
+            let user_count =
+                NonZeroUsize::new(users).ok_or(ScenarioBuildError::WalletUsersZero { users })?;
+            let total_funds = DEFAULT_FUNDS_PER_WALLET.checked_mul(users as u64).ok_or(
+                ScenarioBuildError::WalletFundsOverflow {
+                    users,
+                    per_wallet: DEFAULT_FUNDS_PER_WALLET,
+                },
+            )?;
+
+            let wallet = WalletConfig::uniform(total_funds, user_count);
+            topology = topology.with_wallet_config(wallet);
+        }
+
         let generated = topology.build()?;
         let duration = enforce_min_duration(&generated, duration);
         let run_metrics = RunMetrics::from_topology(&generated, duration);
-        initialize_components(&generated, &run_metrics, &mut workloads, &mut expectations);
+        initialize_components(&generated, &run_metrics, &mut workloads, &mut expectations)?;
+        let workloads: Vec<Arc<dyn Workload>> = workloads.into_iter().map(Arc::from).collect();
 
         info!(
             validators = generated.validators().len(),
@@ -289,12 +315,6 @@ impl<Caps> TopologyConfigurator<Caps> {
     /// Finalize and return the underlying scenario builder.
     #[must_use]
     pub fn apply(self) -> Builder<Caps> {
-        let participants = self.validators + self.executors;
-        assert!(
-            participants > 0,
-            "topology must include at least one node; call validators()/executors() before apply()"
-        );
-
         let mut config = TopologyConfig::with_node_numbers(self.validators, self.executors);
         if self.network_star {
             config.network_params.libp2p_network_layout = Libp2pNetworkLayout::Star;
@@ -316,43 +336,46 @@ impl Builder<()> {
 fn initialize_components(
     descriptors: &GeneratedTopology,
     run_metrics: &RunMetrics,
-    workloads: &mut [Arc<dyn Workload>],
+    workloads: &mut [Box<dyn Workload>],
     expectations: &mut [Box<dyn Expectation>],
-) {
-    initialize_workloads(descriptors, run_metrics, workloads);
-    initialize_expectations(descriptors, run_metrics, expectations);
+) -> Result<(), ScenarioBuildError> {
+    initialize_workloads(descriptors, run_metrics, workloads)?;
+    initialize_expectations(descriptors, run_metrics, expectations)?;
+    Ok(())
 }
 
 fn initialize_workloads(
     descriptors: &GeneratedTopology,
     run_metrics: &RunMetrics,
-    workloads: &mut [Arc<dyn Workload>],
-) {
+    workloads: &mut [Box<dyn Workload>],
+) -> Result<(), ScenarioBuildError> {
     for workload in workloads {
-        let inner =
-            Arc::get_mut(workload).expect("workload unexpectedly cloned before initialization");
-
-        debug!(workload = inner.name(), "initializing workload");
-        if let Err(err) = inner.init(descriptors, run_metrics) {
-            panic!("workload '{}' failed to initialize: {err}", inner.name());
-        }
+        debug!(workload = workload.name(), "initializing workload");
+        workload.init(descriptors, run_metrics).map_err(|source| {
+            ScenarioBuildError::WorkloadInit {
+                name: workload.name().to_owned(),
+                source,
+            }
+        })?;
     }
+    Ok(())
 }
 
 fn initialize_expectations(
     descriptors: &GeneratedTopology,
     run_metrics: &RunMetrics,
     expectations: &mut [Box<dyn Expectation>],
-) {
+) -> Result<(), ScenarioBuildError> {
     for expectation in expectations {
         debug!(expectation = expectation.name(), "initializing expectation");
-        if let Err(err) = expectation.init(descriptors, run_metrics) {
-            panic!(
-                "expectation '{}' failed to initialize: {err}",
-                expectation.name()
-            );
-        }
+        expectation
+            .init(descriptors, run_metrics)
+            .map_err(|source| ScenarioBuildError::ExpectationInit {
+                name: expectation.name().to_owned(),
+                source,
+            })?;
     }
+    Ok(())
 }
 
 fn enforce_min_duration(descriptors: &GeneratedTopology, requested: Duration) -> Duration {